Fabrication of platinum counter electrodes for bifacial dye-sensitized solar cells

ABSTRACT

A fabrication method for a flexible bifacial dye-sensitized solar cell is described. The method involves forming a flexible counter electrode of crystalline Pt nanoparticles on a first conductive layer by irradiating a precursor solution with a UV lamp. A flexible photoanode is formed by applying metal oxide particles to a second conductive layer, and then the solar cell is constructed by sandwiching an electrolyte between the counter electrode and photoanode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority from U.S. ProvisionalApplication No. 62/654,033 filed Apr. 6, 2018, which is incorporatedherein by reference in its entirety.

STATEMENT OF ACKNOWLEDGEMENT

The inventors are thankful to King Fahd University of Petroleum &Minerals (KFUPM) and Center for Excellence in Nanotechnology (CENT),KFUPM, Saudi Arabia for support of this research.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to a method of making a platinum counterelectrode for a flexible bifacial dye-sensitized solar cell.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Third generation solar cells such as dye-sensitized solar cells (DSSCs),quantum-dots sensitized solar cells (QDSSCs), and recently perovskitesolar cells (PSCs) have generated enormous research interest as they areexpected to ultimately rival and possibly replace silicon solar cells inthe pursuit of renewable and clean energy utilizing the abundant energyof the sun. The research interests attracted by these third generationsolar cells are due to their solution processing capabilities, lowcosts, easy fabrication techniques, efficient performance, and potentialfor application in flexible devices. See Yun, S., Hagfeldt, A. & Ma, T.Pt-free counter electrode for dye-sensitized solar cells with highefficiency. Adv. Mater. 26, 6210-6237 (2014); QDSSCs ReviewElsevier.pdf.; Duan, J., Zhang, H., Tang, Q., He, B. & Yu, L. Recentadvances in critical materials for quantum dot-sensitized solar cells: areview. J. Mater. Chem. A 3, 17497-17510 (2015); Docampo, P., Ball, J.M., Darwich, M., Eperon, G. E. & Snaith, H. J. polymer substrates. Nat.Commun. 4, 1-6 (2013); and Giacomo, F. Di et al. Flexible PerovskitePhotovoltaic Modules and Solar Cells Based on Atomic Layer DepositedCompact Layers and UV-Irradiated TiO₂ Scaffolds on Plastic Substrates.1-9 (2015). doi:10.1002/aenm.201401808, each incorporated herein byreference in their entirety. Since the work of Gratzel in 1991, variouscomponents of DSSCs such as the substrates, photoanodes, sensitizers(dyes), electrolytes, and counter electrodes (CEs) have continued to beresearched for new materials, synthesis methods, and fabricationtechniques. See Oregan, B. & Gratzel, M. a Low-Cost, High-EfficiencySolar-Cell Based on Dye-Sensitized Colloidal TiO₂ Films. Nature 353,737-740 (1991), incorporated herein by reference in its entirety. N-typephotoanodes with molecularly engineered dye have reached a record powerconversion efficiency (PCE) of 13%. See Mathew, S. et al. Dye-sensitizedsolar cells with 13% efficiency achieved through the molecularengineering of porphyrin sensitizers. Nat. Chem. 6, 242-247 (2014),incorporated herein by reference in its entirety. TiO₂ remains the mostcommonly used n-type semiconducting photoanode. See Mathew, S. et al.;Lee, K. M. et al. Dye-sensitized solar cells with a micro-porous TiO₂electrode and gel polymer electrolytes prepared by in situ cross-linkreaction. Sol. Energy Mater. Sol. Cells 93, 2003-2007 (2009); OparaKrašovec, U., Berginc, M., Hočevar, M. & Topič, M. Unique TiO₂ paste forhigh efficiency dye-sensitized solar cells. Sol. Energy Mater. Sol.Cells 93, 379-381 (2009); So, S., Hwang, I. & Schmuki, P. HierarchicalDSSC structures based on ‘single walled’ TiO₂ nanotube arrays reach aback-side illumination solar light conversion efficiency of 8%. EnergyEnviron. Sci. 8, 849-854 (2015); Performance enhancement ofdye-sensitized solar cells based TiO₂ thick mesoporous photoanodes bymorphological manipulation. (2015); and TiO₂ low index, eachincorporated herein by reference in their entirety. ZnO, photoactivedoped metal oxides, and various nanocomposite materials have beenreported as photoanodes in DSSCs. See Alpuche-Aviles, M. A. & Wu, Y.Photoelectrochemical Study of the Band Structure of Zn₂SnO₄ Prepared bythe Hydrothermal Method. J. Am. Chem. Soc. 131, 3216-3224 (2009); Han,B. S. et al. Room Temperature Deposition of Crystalline Nanoporous ZnONanostructures for Direct Use as Flexible DSSC Photoanode. NanoscaleRes. Lett. 11, 221 (2016); and Thapa, A. et al. TiO₂ coated urchin-likeSnO₂ microspheres for efficient dye-sensitized solar cells. Nano Res. 7,1154-1163 (2014), each incorporated herein by reference in theirentirety. Ruthenium (Ru) based sensitizers such as N3 and N719, amongothers, have become the de facto dye materials for DSSCs. See Jo, Y. etal. A novel dye coating method for N719 dye-sensitized solar cells.Electrochim. Acta 66, 121-125 (2012), incorporated herein by referencein its entirety. Liquid electrolytes are usually comprised ofiodide/triiodide (I⁻/I³⁻) and cobalt complex (Co²⁺/Co³⁺) redox couples.See Bella, F., Galliano, S., Gerbaldi, C. & Viscardi, G. Cobalt-basedelectrolytes for dye-sensitized solar cells: Recent advances towardsstable devices. Energies 9, 1-22 (2016); and Wu. J. el al. Electrolytesin dye-sensitized solar cells. Chem. Rev. 115, 2136-2173 (2015), eachincorporated herein by reference in their entirety.Highly-electrocatalytic materials like platinum (Pt) are used to reduceI³⁻ to I⁻ at the CE surface in order to sustain the flow of current andregenerate molecules of the oxidized sensitizer in DSSC devices. See Li,L.-L., Wu, H.-H., Tsai, C.-H. & Wei-Guang Diau, E. Nanofabrication ofuniform and stabilizer-free self-assembled platinum monolayers ascounter electrodes for dye-sensitized solar cells. NPG Asia Mater. 6,e118 (2014); and Hsieh, T. Y. et al. A room-temperature process forfabricating a nano-Pt counter electrode on a plastic substrate forefficient dye-sensitized cells. J. Power Sources 283, 351-357 (2015),each incorporated herein by reference in their entirety.

The CE is a focal point of research for the improvement and advancementof DSSCs. Various efforts have investigated other non-platinumelectrocatalytic materials as well as new techniques for the fabricationof Pt CE. See Yun, S., Hagfeldt, incorporated herein by reference in itsentirety. Some of the alternative materials that have been investigatedand reported include polymeric conducting materials such aspoly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS),carbon materials such as carbon soot, graphene, carbon nanotube (CNT),carbon nanofiber (CNF) and graphite, inorganic semiconductingchalcogenide compounds such as NiS, CoS, and CoSe, platinic compositematerials, and other electrocatalytic composite materials. See Yun, S.,Hagfeldt; Anothumakkool, B. et al. Pt- and TCO-Free Flexible Cathode forDSSC from Highly Conducting and Flexible PEDOT Paper Prepared via inSitu Interfacial Polymerization. ACS Appl. Mater. Interfaces 8, 553-562(2016); Zhao, X. et al. A novel hierarchical Pt- and FTO-free counterelectrode for dye-sensitized solar cell. Nanoscale Res. Lett. 9, 202(2014); Yue, G. et al. A dye-sensitized solar cell based on PEDOT:PSScounter electrode. Chinese Sci. Bull. 58, 559-566 (2013); Wei, W., Wang,H. & Hu, Y. H. A review on PEDOT-based counter electrodes fordye-sensitized solar cells. Int. J. Energy Res. 38, 1099-1111 (2014);Ali, A. et al. Flexible, Low Cost, and Platinum-Free Counter Electrodefor Efficient Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 8,25353-25360 (2016); Ali, I., Chul, K., Ayoub, A., Bilal, M. & Hoon, S.Graphene coated cotton fabric as textile structured counter electrodefor DSSC. Electrochim. Acta 173, 164-171 (2015); Choi, H., Kim, H.,Hwang, S., Han, Y. & Jeon, M. Graphene counter electrodes fordye-sensitized solar cells prepared by electrophoretic deposition. J.Mater. Chem. 21, 7548-7551 (2011); Fu, L. & Yu, a M. Carbon NanotubesBased Thin Films: Fabrication, Characterization and Applications. Rev.Adv. Mater. Sci 36, 40-61 (2014); Hsu, S.-H. et al. Platinum-FreeCounter Electrode Comprised of Metal-Organic-Framework (MOF)-DerivedCobalt Sulfide Nanoparticles for Efficient Dye-Sensitized Solar Cells(DSSCs). Sci. Rep. 4, 6983 (2014); Chen, H. Y. et al. Highly catalyticcarbon nanotube/Pt nanohybrid-based transparent counter electrode forefficient dye-sensitized solar cells. Chem.—An Asian J. 7, 1795-1802(2012); Yue, G. et al. A highly efficient flexible dye-sensitized solarcell based on nickel sulfide/platinum/titanium counter electrode.Nanoscale Res. Lett. 10, 1 (2015); Lee, C. P. et al. Economicallow-light photovoltaics by using the Pt-free dye-sensitized solar cellwith graphene dot/PEDOT: PSS counter electrodes. Nano Energy 18, 109-117(2015); and Li, C. T., Lee, C. P., Li, Y. Y., Yeh, M. H. & Ho, K. C. Acomposite film of TiS₂/PEDOT:PSS as the electrocatalyst for the counterelectrode in dye-sensitized solar cells. J. Mater. Chem. A 1,14888-14896 (2013), each incorporated herein by reference in theirentirety. Polymeric conducting materials and carbon materials have theadvantages of low costs, solution processing, and a low temperaturefabrication requirement. However, Pt has consistently shown excellentelectrocatalytic performance and holds the record of the highest PCE forDSSCs. See Li, L.-L. et al. incorporated herein by reference in itsentirety. Pt CEs are usually fabricated at an elevated temperature of450° C. from platinic acid (H₂PtCl₆) precursor or vacuum sputtered froma Pt target. See Lan, Z., Wu, J., Lin, J. & Huang, M. Morphologycontrollable fabrication of Pt counter electrodes for highly efficientdye-sensitized solar cells. J. Mater. Chem. 22, 3948 (2012); Moraes, R.S., Saito, E., Leite, D. M. G., Massi, M. & da Silva Sobrinho, A. S.Optical, electrical and electrochemical evaluation of sputtered platinumcounter electrodes for dye-sensitized solar cells. Appl. Surf Sci. 364,229-234 (2016); and lefanova, A. el al. Low Cost Platinum CounterElectrode for Dye-Sensitized Solar Cells. 2716-2719 (2013), eachincorporated herein by reference in their entirety. Thermaldecomposition of H₂PtCl₆ for the fabrication of Pt CE is not suitablefor material with lower thermal stability at the required elevatedtemperature for the synthesis of Pt. Hence, flexible Pt CE on conductivepolyethylene naphtholate (PEN), polyethylene terephthalate (PET), andtextiles cannot be achieved through a thermal decomposition process. SeeLi, L.-L. et al; Hsieh, T. Y. et al.; and Gong, Y. et al. Simple Methodfor Manufacturing Pt Counter Electrodes on Conductive Plastic Substratesfor Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 5, 795-800(2013), each incorporated herein by reference in their entirety.Sputtering deposition, on the other hand, results in wastage of materialduring deposition process, which limits its use for large scaleproduction as it is not cost effective. See Li, L.-L. et al; Hsieh, T.Y. et al.; and Gong, Y. et al, each incorporated herein by reference intheir entirety.

Researchers have reported several attempts at fabricating Pt CEs at lowtemperature. Electrodeposition technique is one of such methods employedin the fabrication of Pt CEs at low temperature. This method, whichtakes place at room temperature, involves three electrodes configuredwith a transparent conductive oxide (TCO) substrate acting as theworking electrode and with the electrolyte solution containing platinicacid. See Zhang, D., Chang, W. C., Okajima, T. & Ohsaka, T.Electrodeposition of platinum nanoparticles in a room-temperature ionicliquid. Langmuir 27, 14662-8 (2011); Zhang, L., Fang, Z., Zhao, G. C. &Wei, X. W. Electrodeposited platinum nanoparticles on the multi-walledcarbon nanotubes and its electrocatalytic for nitric oxide. Int. J.Electrochem. Sci. 3, 746-754 (2008); and Domínguez-Domínguez, S.,Arias-Pardilla, J., Berenguer-Murcia, Á., Morallón, E. & Cazorla-Amorós,D. Electrochemical deposition of platinum nanoparticles on differentcarbon supports and conducting polymers. J. Appl. Electrochem. 38,259-268 (2008), each incorporated herein by reference in their entirety.A cyclic voltammetry process is then performed using an electrochemicalsystem. Electrophoretic deposition was used by Yin et al., who prepareda H₂PtCl₆ glycol solution and preheated it under stirring for 6 h in anatmosphere of argon. ITO-PEN substrates were immersed in the resultingPt-colloid and driven by a D.C. field of 1.6 V·cm⁻¹. The Pt coatedelectrode was washed with deionized (DI) water and ethanol before beingpost thermally treated at 60° C. for 30 min. See Yin, X., Xue, Z. & Liu,B. Electrophoretic deposition of Pt nanoparticles on plastic substratesas counter electrode for flexible dye-sensitized solar cells. J. PowerSources 196, 2422-2426 (2011), incorporated herein by reference in itsentirety. However, both electrodeposition and electrophoretic depositionmethods have the shortcoming of large Pt loading in the electrochemicalbaths making them unfeasible for commercial production.

Some other alternative methods have however been reported. Chemicalwet-chemistry reduction has been utilized for the fabrication of Pt CEsfrom H₂PtCl₆, employing acidic reducing agents without subsequenttreatment. Matoh et al. employed a chemical reduction method withprepared H₂PtCl₆ in ethanol for the synthesis of nanostructured metallicPt. See Matoh, L., Kozjek Škofic, I., Čeh, M. & Bukovec, N. A novelmethod for preparation of a platinum catalyst at low temperatures. J.Mater. Chem. A 1, 1065 (2013), incorporated herein by reference in itsentirety. The ethanolic Pt precursor was either spin-coated ordrop-coated on fluorine doped tin oxide (FTO) glass electrodes or indiumdoped tin oxide (ITO) PET flexible substrates and dried at roomtemperature. The coated surfaces were then treated with a gaseous formicacid reducing agent at a temperature of 100° C. for a period of 15minutes.

In another example, Hseih et al. used a modified chemical reductionmethod to fabricate Pt CEs. See Hsieh, T. Y. et al., incorporated hereinby reference in its entirety. Polyvinylpyrrolidone (PVP) served assurfactant, NaHBr₄ as a reducing agent, and NaOH was used to achieve aneutral platinic precursor. UV-ozone treatment was used to decompose thesurfactant after deposition on FTO or ITO-PEN. Polyol reductiontechnique is a facile method of synthesis of Pt from H₂PtCl₆ wherebyethylene glycol (EG) is used as a reducing agent. Mei et al. fabricatedPt CEs using EG solution of H₂PtCl₆.6H₂O. See Mei, X.-G., Fan, B.-H.,Sun, K. & Ouyang, J.-Y. High-performance dye-sensitized solar cells withnanomaterials as counter electrode. Proc. SPIE 7411, 74110A/1-74110A/9(2009), incorporated herein by reference in its entirety. The depositedprecursor was thermally treated at 180° C. The synthesized Pt on thesubstrates exhibited dense and porous Pt structures. The earlierresulted from growth of Pt on the substrates following the reduction,while the latter is due to Pt nanoparticle precipitation.

In another example, Li et al. used a similar polyol method withmodification of the pH of the H₂PtCl₆ and with preheating the precursorsolution at 110° C. for 30 mins. See Li, L.-L. et al., incorporatedherein by reference in its entirety. They also pretreated the substrateswith ‘piranha’ solution and 3-mercaptopropyl(trimethoxysilane) (MPTMS)to produce a thiol-functionalized silane self-assembled monolayer (SAM)film on the conductive substrates. The as-prepared functionalizedsubstrates were soaked in the platinic EG solution for 12 h and rinsedwith ethanol to eliminate undesirable residues and dried in a nitrogenenvironment.

Moreover, the high transmittances recorded for the photofabricated PtCEs make them suitable for use in bifacial DSSCs. Bifacial DSSCs can bedeployed as building windows and integrated electronic devices. SeeCarretero-palacios, S. et al. In Situ Prepared Transparent PolyanilineElectrode and Its Application in Bifacial. 1953-1961 (2016).doi:10.1039/C5TA10091G, incorporated herein by reference in itsentirety.

In view of the forgoing, one objective of the present invention is toprovide a method of fabricating a flexible bifacial dye-sensitized solarcell.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to a methodof fabricating a flexible bifacial dye-sensitized solar cell. The methodinvolves a step of depositing a solution comprising a platinum precursorand an alcohol onto a first conductive layer of a first flexiblesubstrate to produce a coated substrate. Then, the coated substrate isirradiated with a UV lamp to form crystalline platinum nanoparticles onthe first substrate, thereby forming a flexible Pt counter electrode.Then, a metal oxide is separately applied on a second conductive layerof a second flexible substrate to form a metal oxide coated substrate.This metal oxide coated substrate comprises metal oxide particles on thesecond conductive layer of the second substrate. Then, a dye isdeposited on the metal oxide particles, which forms a flexiblephotoanode. Finally, an electrolyte is sandwiched between the flexiblePt counter electrode and the flexible photoanode, thereby fabricatingthe flexible bifacial dye-sensitized solar cell. Within the flexiblebifacial dye-sensitized solar cell, the electrolyte forms a firstelectrical contact to the dye and to the metal oxide particles of thephotoanode, and the electrolyte forms a second electrical contact to theplatinum nanoparticles of the flexible Pt counter electrode.

In one embodiment of the method, the Pt nanoparticles have an averagediameter of 100-450 nm.

In one embodiment of the method, the Pt nanoparticles have a granularshape.

In one embodiment of the method, the platinum precursor is H₂PtCl₆,H₂PtCl₄, (NH₄)₂PtCl₆, or K₂PtCl₆.

In one embodiment of the method, the alcohol has a boiling point of 83°C. or lower at atmospheric pressure.

In a further embodiment, the alcohol is methanol, ethanol, orisopropanol.

In one embodiment of the method, the solution is deposited by droppingthe solution onto the first conductive layer.

In one embodiment of the method, the first conductive layer and thesecond conductive layer are transparent and are each independentlyselected from the group consisting of ITO, FTO, AZO, GZO, IZO, IZTO,IAZO, IGZO, IGTO, and ATO.

In one embodiment of the method, wherein the irradiating is done for aperiod of 45-75 minutes and at an average intensity of 0.1-10 W·cm⁻².

In one embodiment of the method, the first flexible substrate and thesecond flexible substrate each independently comprise a plastic selectedfrom the group consisting of poly(ethylene terephthalate), poly(ethylenenaphthalate), polycarbonate, polypropylene, polyimide, triacetylcellulose, polyethersulfone, cyclo olefin copolymer, and polyarylite.

In one embodiment of the method, the irradiating does not raise asurface temperature of the first flexible substrate above 50° C.

In a further embodiment, the irradiating does not raise a surfacetemperature of the first flexible substrate above 40° C.

In one embodiment of the method, the electrolyte comprises iodide.

In one embodiment of the method, the metal oxide is at least oneselected from the group consisting of ZnO, TiO₂, SnO, Fe₂O₃, WO₃, CeO₂,BiVO₄, SrTiO₃, and BaTiO₃.

In one embodiment of the method, the metal oxide is ZnO.

In one embodiment of the method, the metal oxide is applied in the formof a paste which is heated at a temperature of 100-150° C. to form themetal oxide coated substrate.

In a further embodiment, the paste comprises butanol.

In one embodiment of the method, the flexible bifacial dye-sensitizedsolar cell has a percentage ratio of rear illumination efficiency tofront illumination efficiency of 45-95%.

In one embodiment of the method, the flexible bifacial dye-sensitizedsolar cell under AM1.5 irradiation applied as front or rear illuminationhas an open circuit voltage in a range of 0.4-1.0 V.

In one embodiment of the method, the flexible bifacial dye-sensitizedsolar cell under AM1.5 irradiation applied as front illumination has ashort circuit density of 5.0-17.0 mA/cm².

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A is a schematic showing a photofabriation process and theassembled DSSC, with directions of front and rear illumination.

FIG. 1B shows transmittance spectra of photofabricated Pt CEs(Pt-EG-FTO) with different UV irradiation times and bare FTO forcomparison.

FIG. 1C shows transmittance spectra of a UV irradiated bare FTO glass atdifferent irradiation times.

FIG. 1D shows the UV irradiation time versus resistance of a bare FTO.

FIG. 1E shows XPS survey spectra of a Pt-EG-FTO sample at differentirradiation times.

FIG. 1F shows XPS spectra comparing Pt 4f peaks of a photofabricated PtCE (Pt-EG-FTO) at different irradiation times.

FIG. 2A is a scanning emission microscopy (SEM) image of aphotofabricated Pt CE with 2 h UV irradiation time.

FIG. 2B is a SEM image of a photofabricated Pt CE with 1 h UVirradiation time.

FIG. 2C is a SEM image of a photofabricated Pt CE with 30 min UVirradiation time.

FIG. 2D shows a cyclic voltammetry (CV) scan of photofabricated Pt CEs(Pt-EG-FTO) at irradiation times of 2 h, 1 h, and 30 min.

FIG. 2E shows Nyquist plots of photofabricated Pt CEs (Pt-EG-FTO) atdifferent irradiation times of 2 h, 1 h, and 30 min.

FIG. 2F is the circuit model used for fitting the EIS Nyquist plots ofthe samples.

FIG. 2G shows Tafel plots of Pt-EG-FTO CEs with UV irradiation time of 2h, 1 h and 30 min.

FIG. 3A shows transmittance spectra of Pt CEs (Pt-EtOH-FTO) withdifferent irradiation times.

FIG. 3B shows XPS survey spectra of photofabricated Pt CEs (Pt-EtOH-FTO)with different UV irradiation time.

FIG. 3C shows XPS spectra comparing Pt 4f peaks of a photofabricated PtCE (Pt-EtOH-FTO) at different irradiation times.

FIG. 4A is a scanning emission microscopy (SEM) image of aphotofabricated Pt CE (Pt-EtOH-FTO) with 1 h UV irradiation time.

FIG. 4B is a SEM image of a photofabricated Pt CE (Pt-EtOH-FTO) with 30min UV irradiation time.

FIG. 4C is a SEM image of a photofabricated Pt CE (Pt-EtOH-FTO) with 15min UV irradiation time.

FIG. 4D shows a cyclic voltammetry (CV) scan of photofabricated Pt CEs(Pt-EtOH-FTO) at irradiation times of 1 h, 30 min, and 15 min.

FIG. 4E shows Nyquist plots of photofabricated Pt CEs (Pt-EtOH-FTO) atdifferent irradiation times of 1 h, 30 min, and 15 min.

FIG. 4F shows Tafel plots of Pt-EtOH-FTO CEs with UV irradiation time of1 h, 30 min, and 15 min.

FIG. 5A shows the transmittance spectra of a photofabricated Pt flexibleCE and a bare PET-ITO substrate.

FIG. 5B shows an SEM image of photofabricated Pt CE (Pt-EtOH-ITO-PET)with 1 h UV irradiation time.

FIG. 5C shows a CV scan measurement of a photofabricated Pt CE(Pt-EtOH-PET-ITO) at an irradiation time of 1 h as compared with a barePET-ITO substrate.

FIG. 5D shows a Nyquist plot of a photofabricated Pt CE(Pt-EtOH-PET-ITO) at an irradiation time of 1 h.

FIG. 5E shows a Tafel plot of a flexible Pt-EtOH-ITO-PET CE with 1 h UVirradiation time.

FIG. 6A is a photograph of a typical assembled cell samples usingphotofabricated Pt CEs on FTO substrates.

FIG. 6B is a flexible DSSC using a photofabricated Pt CE on a PET-ITOsubstrate.

FIG. 7A shows a front illumination I-V curve of DSSCs based onphotofabricated Pt CEs (Pt-EG-FTO).

FIG. 7B shows a front illumination I-V curve of DSSCs withphotofabricated Pt CEs (Pt-EtOH-FTO).

FIG. 7C shows a front illumination I-V curve of a flexible DSSC with aphotofabricated Pt flexible CE (Pt-EtOH-ITO-PET).

FIG. 8A shows a rear (or back) illumination I-V curve of DSSCs based onphotofabricated Pt CEs (Pt-EG-FTO).

FIG. 8B shows a rear illumination I-V curve of DSSCs withphotofabricated Pt CEs (Pt-EtOH-FTO).

FIG. 8C shows a rear illumination I-V curve of a flexible DSSC withphotofabricated Pt flexible CE (Pt-EtOH-ITO-PET).

FIG. 9A shows a photograph of a typical transparent photofabricated PtCE.

FIG. 9B shows a photograph of photofabricated Pt CEs (upper left, upperright, and bottom right), and an opaque thermally fabricated Pt CE(bottom left, darker square).

FIG. 9C shows a photograph of photofabricated Pt CEs (upper left, upperright, and bottom right), and a thermally fabricated Pt CE (bottom left,darker square).

FIG. 9D shows I-V curves of front and rear illuminated DSSCs utilizingthermally fabricated Pt CE @ 450° C.

FIG. 9E shows a transmittance spectrum of thermally fabricated Pt CE @450° C.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure will be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of “one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. It will be further understood thatthe terms “comprises” and/or “comprising,” when used in thisspecification, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

As used herein, the words “about,” “approximately,” or “substantiallysimilar” may be used when describing magnitude and/or position toindicate that the value and/or position described is within a reasonableexpected range of values and/or positions. For example, a numeric valuemay have a value that is +/−0.1% of the stated value (or range ofvalues), +/−1% of the stated value (or range of values), +/−2% of thestated value (or range of values), +/−5% of the stated value (or rangeof values), +/−10% of the stated value (or range of values), +/−15% ofthe stated value (or range of values), or +/−20% of the stated value (orrange of values). Within the description of this disclosure, where anumerical limit or range is stated, the endpoints are included unlessstated otherwise. Also, all values and subranges within a numericallimit or range are specifically included as if explicitly written out.

As used herein, “compound” is intended to refer to a chemical entity,whether as a solid, liquid, or gas, and whether in a crude mixture orisolated and purified.

As used herein, “composite” refers to a combination of two or moredistinct constituent materials into one. The individual components, onan atomic level, remain separate and distinct within the finishedstructure. The materials may have different physical or chemicalproperties, that when combined, produce a material with characteristicsdifferent from the original components. In some embodiments, a compositemay have at least two constituent materials that comprise the sameempirical formula but are distinguished by different densities, crystalphases, or a lack of a crystal phase (i.e. an amorphous phase).

For polygonal shapes, the term “diameter,” as used herein, and unlessotherwise specified, refers to the greatest possible distance measuredfrom a vertex of a polygon through the center of the face to the vertexon the opposite side. For a circle, an oval, and an ellipse, “diameter”refers to the greatest possible distance measured from one point on theshape through the center of the shape to a point directly across fromit.

The present disclosure is intended to include all hydration states of agiven compound or formula, unless otherwise noted or when heating amaterial. For example, H₂PtCl₆.6H₂O includes anhydrous H₂PtCl₆,[H₃O]₂[PtCl₆](H₂O)_(x), H₂PtCl₆.6H₂O, and any other hydrated forms ormixtures.

In addition, the present disclosure is intended to include all isotopesof atoms occurring in the present compounds and complexes. Isotopesinclude those atoms having the same atomic number but different massnumbers. By way of general example, and without limitation, isotopes ofhydrogen include deuterium and tritium. Isotopes of carbon include ¹³Cand ¹⁴C. Isotopes of nitrogen include ¹⁴N and ¹⁵N. Isotopes of oxygeninclude ¹⁶O, ¹⁷O, and ¹⁸O. Isotopes of zinc include ⁶⁴Zn, ⁶⁶Zn, ⁶⁷Zn,⁶⁸Zn, and ⁷⁰Zn. Isotopes of platinum include ¹⁹⁰Pt, ¹⁹²Pt, ¹⁹³Pt, ¹⁹⁴Pt,¹⁹⁵Pt, ¹⁹⁶Pt, and ¹⁹⁸Pt. Isotopically-labeled compounds of thedisclosure may generally be prepared by conventional techniques known tothose skilled in the art or by processes analogous to those describedherein, using an appropriate isotopically-labeled reagent in place ofthe non-labeled reagent otherwise employed.

According to a first aspect, the present application relates to a methodof fabricating a flexible bifacial dye-sensitized solar cell. Adye-sensitized solar cell (DSSC) refers to an electrical device thatconverts the energy of light directly into electricity (i.e. aphotoelectric conversion mechanism) by a photovoltaic effect. Adye-sensitized solar cell mainly includes a transparent substrate (oranode), a semiconductor layer, a photo-sensitizing dye, an electrolyte,and a counter electrode (or cathode). A flexible anode and counterelectrode may enable the DSSC to be flexible. The photoelectricconversion mechanism thereof is as follows. When sunlight reaches theDSSC, the dye molecules absorb energy from sunlight, and electrons inthe dye molecules are excited and transit to an excited state. Theelectrons in the excited state are rapidly injected into a conductionband of the semiconductor layer, and thus the dye molecules aretransformed to an oxidized state due to a loss of electrons. Theelectrons injected into the conduction band of the semiconductor layer,instantly reach a contact interface between the semiconductor layer andthe transparent substrate of the anode, accumulate on the anode, andflow to the counter electrode (or cathode) through an outer circuitincluding a load. The dye molecules in the oxidized state acceptelectrons from an electron donor in the electrolyte and return to theground state, thereby regenerating the dye molecules. After losing itselectron, the electron donor in the electrolyte diffuses to the counterelectrode and accepts an electron and is reduced. In this way, aphotoelectric chemical reaction cycle is completed. The counterelectrode is mainly used for collecting electrons and accelerating theelectron exchange rate between the electrolyte and counter electrode.

With a transparent cathode, bifacial solar cells can absorb light fromboth the front and rear sides. Hence, they may produce more electricitythan conventional monofacial solar cells. As used herein, the flexiblebifacial dye-sensitized solar cell may also be called a “flexibledye-sensitized solar cell” or a “flexible DSSC,” with the understandingthat the solar cell is bifacial unless noted otherwise. As defined here,the front illumination refers to light that is incident on the frontside, or photoanode side of the DSSC. Rear illumination refers to lightincident on the rear side, or counter electrode side. Rear illuminationgenerally produces less power than front illumination, asrear-illuminated light has to pass through the electrolyte before beingabsorbed by dye.

As described more fully herein, the method of fabricating the flexibleDSSC involves a step of depositing a solution comprising a platinumprecursor and an alcohol onto a first conductive layer of a firstflexible substrate to produce a coated substrate. Then, the coatedsubstrate is irradiated with a UV lamp to form crystalline platinumnanoparticles on the first substrate, thereby forming a flexible Ptcounter electrode. Then, a metal oxide is separately applied on a secondconductive layer of a second flexible substrate to form a metal oxidecoated substrate. This metal oxide coated substrate comprises metaloxide particles on the second conductive layer of the second substrate.Then, a dye is deposited on the metal oxide particles, which forms aflexible photoanode. Finally, an electrolyte is sandwiched between theflexible Pt counter electrode and the flexible photoanode, therebyfabricating the flexible bifacial dye-sensitized solar cell. Within theflexible bifacial dye-sensitized solar cell, the electrolyte forms afirst electrical contact to the dye and to the metal oxide particles ofthe photoanode, and the electrolyte forms a second electrical contact tothe platinum nanoparticles of the flexible Pt counter electrode.

In one embodiment, the first flexible substrate and the second flexiblesubstrate may each independently comprise a plastic selected from thegroup consisting of polyethylene (PE), poly(ethylene terephthalate)(PET), poly(ethylene naphthalate) (PEN), polycarbonate (PC),polypropylene (PP), polyacrylate, poly methyl methacrylate (PMMA),polystyrene (PS), polyimide (PI), triacetyl cellulose (TAC),polyethersulfone, polybutylene terephthalate, cyclo olefin copolymer(COC), and polyarylite. Where the plastic is polyethylene, thepolyethylene may be ultra-high-molecular-weight polyethylene (UHMWPE),ultra-low-molecular-weight polyethylene (ULMWPE), high-molecular-weightpolyethylene (HMWPE), high-density polyethylene (HDPE), high-densitycross-linked polyethylene (HDXLPE), cross-linked polyethylene (PEX orXLPE), medium-density polyethylene (MDPE), linear low-densitypolyethylene (LLDPE), low-density polyethylene (LDPE), very-low-densitypolyethylene (VLDPE), or chlorinated polyethylene (CPE). In someembodiments, combinations of two or more plastics may be used, or theplastics may comprise one of the types mentioned above with the additionof co-polymers such as alpha-olefins, vinyl acetate, or acrylate.Preferably the plastic is PEN, PET, or PP, more preferably the plasticis PET. In one embodiment, the first and second flexible substratescomprise the same plastic, though in other embodiments, they may eachcomprise a different plastic.

The first and/or second flexible substrate may have a thickness of 0.1-4mm, preferably 0.5-3 mm, more preferably 0.8-2 mm. A face of the firstand/or second flexible substrate may have an area of 0.1-2,500 cm²,preferably 0.25-100 cm², more preferably 3.5-36 cm², and may have anaspect ratio (length:width) in a range of 10:1-1:1 preferably 4:1-1:1,more preferably 2:1-1:1. Preferably the first and second flexiblesubstrates have similar areas and similar aspect ratios. Preferably thefirst and second flexible substrates are rectangular, though inalternative embodiments the substrates may be circular, elliptical,rounded, triangular, a non-rectangle quadrilateral, or some otherregular or irregular shape.

As defined here, a substrate being “flexible” means that the substratemay be reversibly deformed against a cylindrical surface having acurvature, κ, of up to 0.1 cm⁻¹, preferably up to 0.25 cm⁻¹, morepreferably up to 0.5 cm⁻¹, even more preferably up to 1 cm⁻¹ withoutcausing stress fracture or permanent deformation. By definition, a curvewith a curvature of κ has a radius of curvature of 1/κ. In someembodiments, the substrate may be deformed with a curvature of more than1 cm⁻¹, for instance, 1.1-1.5 cm⁻¹, or 1.6-2.0 cm⁻¹, without causingfracture or permanent deformation. The flexible bifacial dye-sensitizedsolar cell being flexible means that it can be deformed with a similarcurvature as for the substrate without a loss in function or permanentdeformation after being returned to its original shape. For instance,the flexible bifacial dye-sensitized solar cell may be deformed asdescribed without a loss of conversion efficiency and without the solarcell leaking the electrolyte. Generally, thinner flexible substrates maybe reversibly deformed along a greater curvature than thicker flexiblesubstrates comprising the same material.

In an alternative embodiment, a transparent rigid substrate may be used,where the substrate may comprise plastic, glass, or quartz. Where thesubstrate comprises glass, the glass may be boro-aluminosilicate glass,sodium borosilicate glass, fused-silica glass, soda lime glass, or someother type of glass.

In one embodiment, the first conductive layer and the second conductivelayer are transparent, so that the first flexible substrate and secondflexible substrate are each transparent. As used herein, a materialbeing “transparent” is defined as having an average transmittance of85-100% over a wavelength range of 400-700 nm. Alternatively, a materialbeing transparent may have a transmittance of 90-100% over a wavelengthrange of 400-700 nm. For a flat substrate, this transmittance is of alight path that intersects the two largest faces of the substrate. FIG.9A shows an example of flexible substrates being transparent against abackground of printed text. In an alternative embodiment, at least oneof the flexible substrates or conductive layers may be doped or coatedwith a UV sensitizer dye.

In one embodiment, the conductive layers are transparent eachindependently selected from the group consisting of ITO (indium tinoxide), FTO (fluorine-doped tin oxide), AZO (aluminum-doped zinc oxide),GZO (gallium-doped zinc oxide), IZO (indium zinc oxide), IZTO (indiumzinc tin oxide), IAZO (indium aluminum zinc oxide), IGZO (indium galliumzinc oxide), IGTO (indium gallium tin oxide), and ATO (antimony tinoxide). These layers may be known as transparent conductive films, andin other embodiments, transparent conducting polymers (such as PEDOT) orcarbon nanotubes may be used with or in place of the compoundspreviously mentioned. Preferably the conductive layers are both ITO orboth FTO, and in a preferred embodiment, both conductive layers are ITO.

Indium tin oxide (ITO) is a ternary composition of indium, tin, andoxygen. Depending on the oxygen content, it can either be described as aceramic or alloy. Indium tin oxide, such as that of the presentdisclosure, is usually encountered as an oxygen-saturated compositionwith a formulation of 74 wt % In, 18 wt % O₂, and 8 wt % Sn, eachrelative to a total weight of the composition. In terms of SnO₂ andIn₂O₃, this composition is equivalent to and may be represented as 10 wt% SnO₂ and 90 wt % In₂O₃, both relative to a total weight of thecomposition. ITO is transparent and colorless in thin layers, while inbulk form it is yellowish to grey. In the infrared region of thespectrum it acts as a metal-like mirror. Oxygen unsaturated compositionsare less common and are termed oxygen-deficient ITO. In otherembodiments, the ITO of the present disclosure may compriseoxygen-deficient ITO, or ITO having compositions of In, O, and/or Snthat are different than those in the oxygen-saturated composition.Preferably the ITO of the present disclosure is substantially pure,meaning that the ITO comprises greater than 99.0 wt %, preferablygreater than 99.5 wt %, preferably greater than 99.8 wt % of thecombined weight of indium, tin, and oxide, relative to a total weight ofthe ITO. Similarly, preferably the conductive layers comprise greaterthan 99.0 wt % ITO, preferably greater than 99.5 wt % ITO, each relativeto their total weight.

Indium tin oxide is one of the most widely used transparent conductingoxides because of its two main properties: its electrical conductivityand optical transparency, as well as the ease with which it can bedeposited as a thin film. As with all transparent conducting films, acompromise must be made between conductivity and transparency, sinceincreasing the thickness and increasing the concentration of chargecarriers increases the material's conductivity, but decreases itstransparency.

The conductive layers may be present as a film having an averagethickness of 500 nm-200 μm, preferably 1 μm-100 μm, more preferably 10μm-50 μm. However, in some embodiments, the conductive layers may havean average thickness of less than 500 nm. For instance, the conductivelayers may have an average thickness of 50-500 nm, 80-300 nm, or 100-250nm. Preferably, the conductive layers cover all of one side of eachflexible substrate, or about 100% of the area of one side of each.However, in other embodiments, the area of a conductive layer may onlybe 10-90%, or 20-80% of the total area of the side of the flexiblesubstrate. Here, the conductive layer may be rectangular or patterned ina different shape.

In one embodiment, a surface resistivity of either conductive layer maybe 1-50 Ω/sq, preferably 4-25 Ω/sq, more preferably 5-20 Ω/sq. However,in some embodiments, the surface resistivity may be less than 1 Ω/sq orgreater than 50 Ω/sq.

In one embodiment, either conductive layer may have an area in directcontact with an electrically-conductive material so that the flexibleDSSC may form part of a circuit. An “electrically-conductive material”as defined here is a substance with an electrical resistivity of at most10⁻⁶ Ω·m, preferably at most 10⁻⁷ Ω·m, more preferably at most 10⁻⁸ Ω·mat a temperature of 20-25° C. The electrically-conductive material maybe platinum-iridium alloy, iridium, titanium, titanium alloy, stainlesssteel, gold, cobalt alloy, copper, aluminum, tin, iron, and/or someother metal. In other embodiments, a part of a conductive layer mayextend from the flexible substrate in order to be connected to anelectrically-conductive material. An electrically-conductive materialmay be held in place by solder, a clip, a conductive adhesive, or someother means.

In one embodiment, the platinum precursor may be H₂PtCl₆, H₂PtCl₄,(NH₄)PtCl₆, and/or K₂PtCl₆. In a preferred embodiment, the platinumprecursor is H₂PtCl₆, which is otherwise known as chloroplatinic acid,and may also be written as [H₃O]₂[PtCl₆](H₂O)_(x). In other embodiments,different compounds comprising platinum may be used, for instance,Pt(NH₃)₂(CO₂)₂C₄H₆, Pt(NH₃)₂Cl₂, K₂Pt(CN)₄, Pt(NH₃)₄PtCl₄,Pt(NH₃)₂CO₂CH₂O, C₆H₁₀(NH₂)₂PtC₂O₄, NH₃PtCl₂NC₅H₄CH₃, Pt(C₂H₃O₂)₂,PtBr₂, PtCl₂, K₂PtCl₄, (PtCl(NH₃)₂C₆H₁₂(NH₂)₂)Pt(NH₃)₂(NO₃)₄, Pt(OH)₂,PtS, PtCl₂C₈H₁₂, KPtCl₃C₂H₄, PtO₂, [Pt(NH₃)₄]Cl₂, H₂PtCl₆, PtBr₄, PtCl₄,PtF₄, K₂PtCl₆, Pt(C₂H₃O₂)₂Cl₂NH₃NH₂C₆H₁₁, Na₂PtCl₆, Pt(OH)₄, PtS₂,PtSe₂, XeFPtF₅, PtF₅, O₂PtF₆, XePtF₆, or PtF₆. In an alternativeembodiment, Pt nanoparticles may be deposited, for instance, as a pasteor within a solvent, without a need for a photoreduction step.

In one embodiment, the solution comprising the platinum precursor andthe alcohol is deposited onto the first conductive layer, in order toproduce a coated substrate. This depositing may be done by dropping thesolution. This method may be considered essentially equivalent todripping the solution or applying one or more droplets of the solution.In some embodiments, this method may also be considered “drop casting.”For example, a droplet of the needed volume may be formed on the tip ofa volumetric pipette, and then contacted to and left on the surface ofthe first conductive layer to allow the solution to evaporate.

In other embodiments, the solution may be deposited onto the firstconductive layer by contact printing, spin coating, blade spreading(which may also be called doctor blading or slot coating), screenprinting, spray coating, ink jet printing, bar coating, dip coating,brushing, or immersing. The solution may be dropped onto the firstconductive layer, and then spread. Where the solution is deposited byspin coating, the substrate may be rotated at a rate of 500-8,000 rpm,preferably 1,000-4,000 rpm, more preferably 1,500-2,500 rpm for 10-60 s,preferably 15-45 s, more preferably 20-40 s, and at a ramp rate of100-800 rpm, preferably 200-600 rpm, or about 500 rpm. In embodimentswhere the solution comprises an alcohol or some solvent with a boilingpoint of 83° C. or lower at atmospheric pressure, such as ethanol,isopropanol, or methanol, dropping may be a more effective coatingtechnique than spin coating. In alternative embodiments where thesolution comprises an alcohol or some solvent with a boiling pointgreater than 83° C. at atmospheric pressure, the spin coating mayprovide a more efficient or homogenous coating of the solution thandropping. In some embodiments, a portion of the first conductive layermay be masked or covered, in order to limit the area of the firstconductive layer being contacted.

The first conductive layer may be masked or temporarily covered withtape, a film, or a gasket so that only 5-90%, preferably 10-80%, morepreferably 15-75% of the total area of the first conductive layer isexposed to the solution. In one embodiment, the tape may be SCOTCH tapeby 3M. However, in other embodiments, the no part of the firstconductive layer may be masked or covered, and about 100% of the totalarea of the conductive layer may be

In other embodiments, the solvent may be an organic solvent selectedfrom the group consisting of toluene, tetrahydrofuran, acetic acid,acetone, acetonitrile, butanol, dichloromethane, chloroform,chlorobenzene, dichloroethane, diethylene glycol, diethyl ether,dimethoxy-ethane, dimethyl-formamide, dimethyl sulfoxide, ethanol, ethylacetate, ethylene glycol, heptane, hexamethylphosphoramide,hexamethylphosphorous triamide, methanol, methyl t-butyl ether,methylene chloride, pentane, cyclopentane, hexane, cyclohexane, benzene,dioxane, propanol, isopropyl alcohol, pyridine, triethyl amine,propandiol-1,2-carbonate, ethylene carbonate, propylene carbonate,nitrobenzene, formamide, γ-butyrolactone, benzyl alcohol,n-methyl-2-pyrrolidone, acetophenone, benzonitrile, valeronitrile,3-methoxy propionitrile, dimethyl sulfate, aniline, n-methylformamide,phenol, 1,2-dichlorobenzene, tri-n-butyl phosphate, ethylene sulfate,benzenethiol, dimethyl acetamide, N,N-dimethylethaneamide,3-methoxypropionnitrile, diglyme, cyclohexanol, bromobenzene,cyclohexanone, anisole, diethylformamide, 1-hexanethiol, ethylchloroacetate, 1-dodecanthiol, di-n-butylether, dibutyl ether, aceticanhydride, m-xylene, o-xylene, p-xylene, morpholine, diisopropyletheramine, diethyl carbonate, 1-pentandiol, n-butyl acetate, and1-hexadecanthiol.

The solution may be deposited on the first conductive layer at a volumeper area ratio of 10-300 μL/cm², preferably 15-200 μL/cm², morepreferably 20-130 μL/cm². However, depending on the concentration of Ptand type of solvent, a volume per area ratio of smaller than 10 μL/cm²or greater than 300 μL/cm² may be used. For instance, a solution havinga greater concentration of Pt from the Pt precursor may be applied at asmaller volume to a certain area than a solution having a lowerconcentration, and may result in similar first conductive layers afterthe solvent evaporates. The concentration of the Pt in the solution maybe 0.001-1.0 M, preferably 0.010-0.100 M, more preferably 0.015-0.030 M.The solution may be applied at a layer thickness of 10-500 μm,preferably 20-400 μm, more preferably 30-300 μm. In one embodiment, thesolution consists essentially of the Pt precursor and solvent. Forinstance, the solution may consist of only Pt and ethanol. In oneembodiment, the solution does not contain a surfactant.

In some embodiments, the first conductive layer having the depositedsolution may be left at room temperature for the solvent to evaporate,while in other embodiments, the first flexible substrate may be heatedand/or placed in a desiccator. In other embodiments, rather thandepositing a solution onto the first conductive layer, Pt nanoparticlesmay be directly deposited by a chemical vapor deposition method.

Following the deposition of the solution, the coated substrate is thenirradiated with a UV lamp. Where the Pt precursor is chloroplatinicacid, or some other oxidized form of Pt, the Pt is photoreduced into Ptmetal, preferably in the form of nanoparticles, which forms the flexiblePt counter electrode (CE). The coated substrate may be irradiated withthe UV lamp before the solvent completely evaporates, or when the coatedsubstrate is dry. The coated substrate may be exposed to UV irradiationfrom the UV lamp for 45-75 min, preferably 50-70 min, more preferably55-65 min, and at an average intensity of 0.1-10 W·cm⁻², preferably1.0-5 W·cm⁻², more preferably 1.5-4 W·cm⁻², or about 2 W·cm⁻². However,in some embodiments, the coated substrate may be irradiated for a longertime and at a lower intensity, or for a shorter time with a higheraverage intensity. However, in alternative embodiments, the coatedsubstrate may be irradiated at a lower average intensity for a shortertime or at a higher intensity for a longer time. Preferably, theinstantaneous irradiation power varies from the average irradiationpower by less than 15%, preferably by less than 10%, more preferably byless than 5%. In one embodiment, the irradiating does not raise asurface temperature of the first flexible substrate above 50° C. Here,the coated substrate may not deform or become damaged from overheating,for instance, by melting or softening any plastic within the substratePreferably, the irradiating does not raise a surface temperature of thefirst flexible substrate above 40° C. In another embodiment, theirradiating does not raise a surface temperature of the first flexiblesubstrate above 38° C. Preferably, in these embodiments, the coatedsubstrate is irradiated at room temperature (about 22-25° C.) andwithout forced air cooling. In an alternative embodiment, however,forced air cooling may be used during the irradiating.

The UV lamp may be a tube with, or preferably, without a phosphorcoating. In another embodiment, the UV lamp may be an incandescent lamp,such as a halogen lamp with a fused quartz envelope. In anotherembodiment, the UV lamp may be a gas-discharge lamp, such as a xenon arclamp, an argon arc lamp, a deuterium arc lamp, a mercury-xenon arc lamp,or a metal-halide arc lamp. The UV lamp may emit light in a wavelengthrange of 100-400 nm, preferably 200-400 nm. In other embodiments, the UVlamp may be an excimer lamp, or a UV source such as UV LEDs may be usedinstead of a lamp. In another embodiment, a broadband light source maybe used with filters to select for light in the UV region. In oneembodiment, the coated substrate is not irradiated with a laser. In analternative embodiment, the coated substrate is irradiated with a laser,such as a UV laser. However, in some embodiments, the high, localizedintensity of a UV layer may produce amorphous Pt nanoparticles. Thus, itmay be preferred to irradiate using a UV lamp over a longer time periodand at a lower localized intensity in order to enable the formation ofcrystalline Pt nanoparticles. In one embodiment, the irradiation of theUV lamp may have an average intensity that is substantially similar toits localized or instantaneous intensity on the coated substrate. Thiscontrasts with a UV laser of high intensity which may be scanned acrossthe coated substrate. A UV lamp also provides an advantage of evenirradiation over large surfaces, for instance, if a large coatedsubstrate is irradiated.

Where the flexible Pt counter electrode comprises Pt nanoparticles, inone embodiment, the nanoparticles have an average diameter or longestdimension of 100-450 nm, preferably 150-400 nm, more preferably 200-350nm, more preferably 210-300 nm. However, in some embodiments, thenanoparticles may have an average diameter or longest dimension of lessthan 100 nm or greater than 450 nm.

Preferably, the Pt nanoparticles have a granular shape, as opposed to aflat shape. For instance, the Pt nanoparticles of the SEM images inFIGS. 2B, 4A, 4B, and 5B are granular, while those of FIGS. 2A and 4Care flat. In one embodiment, the Pt nanoparticles may have a granularshape where the distance from the particle centroid (center of mass) toanywhere on the nanoparticle outer surface varies by less than 50%,preferably by less than 40% of the average distance. In one embodiment,the Pt nanoparticles may be more rounded or spherical, where thedistance from the particle centroid to anywhere on the outer surfacevaries by less than 30%, preferably less than 20%, or less than 10%. Insome embodiments, one or more Pt nanoparticles may be shaped likecylinders, boxes, spikes, flakes, plates, ellipsoids, toroids, stars,ribbons, discs, rods, granules, prisms, cones, flakes, platelets,sheets, or some other shape.

Preferably, in one embodiment, the Pt nanoparticles are shaped as thoseshown in FIGS. 2B, 4A, 4B, and/or 5B. For instance, the Pt nanoparticlesmay have sharp edges of a length of 50-450 nm, preferably 100-400 nm,more preferably 120-300 nm, more preferably 140-300 nm. These sharpedges may each be formed by two planar surfaces of a Pt nanoparticleforming an angle of 30°-80°, preferably 35°-75°, more preferably45°-65°. In one embodiment, increasing the length of UV irradiation timeincreases the occurrence of this particle morphology over those ofnanosheets or nanoflakes, such as the nanosheets and nanoflakes shown inFIGS. 2A and 4C.

In one embodiment, the Pt nanoparticles may have an average Wadellsphericity value in a range of 0.3 to 0.9, or 0.3 to 0.8. The Wadellsphericity of a particle is defined by the ratio of the surface area ofa theoretical sphere (which has the same volume as the given particle)to the surface area of the particle. The values of Wadell sphericityrange from 0 to 1, where a value of 1 is a perfect sphere, and particlesbecome less spherical as their sphericity approaches a value of 0. TheWadell sphericity may be approximated by

${\Psi \approx \left( \frac{bc}{a^{2}} \right)^{1/3}},$where a, b, and c are the lengths of the long, intermediate, and shortaxes, respectively, of an individual particle.

In one embodiment, the Pt nanoparticles are monodisperse, having acoefficient of variation or relative standard deviation, expressed as apercentage and defined as the ratio of the particle diameter standarddeviation (σ) to the particle diameter mean (μ), multiplied by 100%, ofless than 25%, preferably less than 10%, preferably less than 8%,preferably less than 6%, preferably less than 5%. In a preferredembodiment, the Pt nanoparticles are monodisperse having a particlediameter distribution ranging from 80% of the average particle diameterto 120% of the average particle diameter, preferably 85-115%, preferably90-110% of the average particle diameter. In another embodiment, the Ptnanoparticles are not monodisperse.

In one embodiment, the Pt nanoparticles may comprise Pt of anon-crystalline or amorphous phase. In this embodiment, the Ptnanoparticles may have a more rounded or more spherical geometry. Thisrounded or spherical geometry may be as described previously, where thedistance from the particle centroid to anywhere on the outer surfacevaries by less than 30%, preferably less than 20%, or less than 10%. Inone embodiment, non-crystalline or amorphous Pt nanoparticles may bepresent as agglomerates.

As mentioned, in one embodiment, the Pt nanoparticles may be present asagglomerates of primary particles. As used herein, the term“agglomerates” refers to a clustered particulate composition comprisingprimary particles, the primary particles being aggregated together insuch a way so as to form clusters thereof, at least 50 volume percent ofthe clusters having a mean diameter that is at least 2 times the meandiameter of the primary particles, and preferably at least 90 volumepercent of the clusters having a mean diameter that is at least 5 timesthe mean diameter of the primary particles. The primary particles may bethe Pt nanoparticles having a mean diameter as previously described, ormay have a smaller mean diameter, such as 40-200 nm, preferably 50-180nm, or 55-170 nm. In one embodiment, the Pt nanoparticles may be roundand agglomerated as those shown in FIG. 5B.

Preferably, the Pt nanoparticles in contact with the conductive layerform an electrically-conductive material with the conductive layer. An“electrically-conductive material” as defined here is a substance withan electrical resistivity of at most 10⁻⁶ Ω·m, preferably at most 10⁻⁷Ω·m, more preferably at most 10⁻⁸ Ω·m at a temperature of 20-25° C.

The method also involves applying a metal oxide on a second conductivelayer of a second flexible substrate to form a metal oxide coatedsubstrate. The metal oxide coated substrate comprises metal oxideparticles on the second conductive layer of the second substrate. In oneembodiment, the metal oxide is at least one selected from the groupconsisting of ZnO, TiO₂, SnO, Fe₂O₃, WO₃, CeO₂, BiVO₄, SrTiO₃, andBaTiO₃. In one embodiment, the metal oxide is ZnO, TiO₂, SnO, Fe₂O₃,WO₃, or CeO₂. In a preferred embodiment, the metal oxide is ZnO. In analternative embodiment, a semiconductive material may be used in placeof a metal oxide, such as GaN, GaAs, MoS₂, WSe₂, MoSe₂, doped silicon,an n-type semiconductor, or some other photocatalytically activecompound. N-type semiconductors are created by doping an intrinsicsemiconductor with an electron donor element. Group IV semiconductorsuse group V atoms as donors and group III atoms as acceptors. For groupIV n-type semiconductors, the intrinsic semiconductor may comprisesilicon or germanium, and the donor element may be phosphorus, arsenic,or antimony. Group III-V semiconductors, the compound semiconductors,use group VI atoms as donors and group II atoms as acceptors. GroupIII-V semiconductors can also use group IV atoms as either donors oracceptors. When a group IV atom replaces the group III element in thesemiconductor lattice, the group IV atom acts as a donor. Conversely,when a group IV atom replaces the group V element, the group IV atomacts as an acceptor. Group IV atoms can act as both donors andacceptors; therefore, they are known as amphoteric impurities. For groupIII-V n-type semiconductors, the intrinsic semiconductor may comprisealuminum phosphide, aluminum arsenide, gallium arsenide, or galliumnitride, and the donor element may be selenium, tellurium, silicon, orgermanium. Other metal oxides that may be used to form the metal oxidecoated substrate include Na₂Ti₆O₁₃, BaTi₄O₉, Ta₂O₅, KTaO₃, K₄Nb₆SO₁₇,and K₂La₂Ti₃O₁₀.

In one embodiment, the metal oxide particles may have an averagediameter or longest dimension of 20-750 nm, preferably 40-500 nm, morepreferably 50-450 nm, even more preferably 55-100 nm. In anotherembodiment, the metal oxide particles may have an average diameter orlongest dimension of 5-200 nm, preferably 10-150 nm, more preferably15-100 nm, even more preferably 20-90 nm. The metal oxide particles mayhave geometries, dispersity, agglomerates, or other morphologies asmentioned previously for the Pt nanoparticles.

In one embodiment, the metal oxide particles may be applied in the formof a paste, and may also be called a slurry or colloidal mixture. Thepaste may have a viscosity of 10 to 8,000 cps (centipoises), preferably100-8,000 cps, more preferably 1,000-8,000 cps. The paste may comprisethe metal oxide in a solvent at a concentration of 0.1-5.5 g/mL,preferably 0.5-3 g/mL, more preferably 0.6-2.0 g/mL. In one embodiment,the solvent is n-butanol, but in other embodiments, the solvent may bean organic solvent as any of those mentioned previously. In oneembodiment, the paste may be applied to the conductive layer to form alayer with a thickness of 10-500 μm, preferably 30-400 μm, morepreferably 40-100 μm. The applying may be by doctor blading, spreading,or some other application technique previously mentioned. In alternativeembodiments, the paste may also comprise a plasticizer (such asdi-2-ethylhexyl phthalate or dibutyl phthalate), a rheology adjustingagent (such as fatty acid amide or fatty acid), a thixotropic agent, athickening agent, or a dispersing agent. In addition, the secondconductive layer may be masked or covered as previously mentioned forthe first conductive layer, in order to minimize the exposed area.

The conductive layer with the paste may be heated at a temperature of100-150° C., preferably 110-140° C., more preferably 115-135° C. to formthe metal oxide coated substrate. In some embodiments, the paste mayonly dried, for instance, by placing in a desiccator, or by heating at atemperature less than 100° C.

The method involves a step of depositing a dye on the metal oxideparticles on the conductive layer, thereby forming a flexiblephotoanode. Preferably the dye is applied as a liquid and may be sprayedor applied by some manner as mentioned previously for the Pt precursorsolution. In another embodiment, the substrate may be submerged orimmersed in a volume of the dye.

With the dye deposition, the metal oxide particles may be coated with alayer of the dye, and the dye molecules may be adsorbed or covalentlybonded to the exterior surface of the metal oxide particles. In someembodiments, there may be parts of the conductive layer that are notcontacted with a metal oxide particle, but are in contact with the dye.Alternatively, there may be parts of the conductive layer that are notin contact with either dye or a metal oxide particle.

In one embodiment, the dye may be a photosensitiveruthenium-polypyridine dye, for instance[Ru(4,4′-dicarboxy-2,2′-bipyridine)₂(NCS)₂] (also known as N3), orcis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(n)bis(tetrabutylammonium) (also known as N719 or RUTHENIZER 535-bisTBA).Other photosensitive ruthenium dyes that may be used include N749, Z907,N712, Ru(byp)₃Cl₂, and Ru(byp)₃Cl₂-imidazolidinone. In a preferredembodiment, the dye is N719. In alternative embodiments, the dye may beRose Bengal (RB), Basic Blue 9, Rhodamine B, methylene blue,meso-tetraphenylporphine (TPP),2-cyano-5-(4-dimethylaminophenyl)penta-2,4-dienoic acid (NKX-2553), orsome other organic dye. In some embodiments, a mixture of two or moredyes may be used. In an alternative embodiment, a juice extract from afruit or vegetable may be used as a dye, such as a juice extract fromblueberries, blackberries, or red cabbage.

Preferably the dye is a photosensitizer, which refers to a compound thatinitiates a light-induced chemical reaction (e.g. photo-oxygenation), byabsorbing electromagnetic radiation, which may be ultraviolet or visiblelight. Accordingly, the photosensitizer enters an excited state whenexposed to light of a specific wavelength, and further reacts witheither a substrate or ground state molecular oxygen, starting a cascadeof energy transfer that results in an oxygenated compound.

The dye may be dissolved in a solvent such as methanol or ethanol at aconcentration of 20 μM-10 mM, preferably 100 μM-1 mM, more preferably200 μM-500 μM. In another embodiment, the dye may be dissolved in thesolvent at a concentration of 0.01-2.00 mg/mL, preferably 0.10-1.00mg/mL, more preferably 0.3-0.5 mg/mL. In another embodiment, a dyeadditive or staining additive may be added to the dye solution, such aschenodeoxycholic acid. In one embodiment, the dye may be left in contactwith the metal oxide particles for a time period of 1-48 h, preferably4-36 h, more preferably 16-30 h, or about 24 h, and at a temperature of4-30° C., preferably 10-28° C., or about room temperature. In oneembodiment, after the contacting, the substrate with the dye may berinsed and/or dried. The drying may be done with a desiccator, an oven,or a heat gun or air dryer.

The fabrication method next involves a step of sandwiching anelectrolyte between the flexible Pt counter electrode and the flexiblephotoanode, so that the electrolyte is in direct contact with the dye ofthe photoanode and the Pt nanoparticles of the Pt counter electrode. Inone embodiment, the electrolyte may comprise an iodide such as aniodide/tri-iodide redox couple in a nitrile solvent. In one embodiment,the electrolyte may comprise an iodide/tri-iodide redox couple at aconcentration of 5-500 mM, preferably 10-100 mM, more preferably 25-75mM. In one embodiment, the nitrile solvent may be acetonitrile,methoxypropionitrile, or propionitrile. Preferably the nitrile solventis acetonitrile. The electrolyte may be present in a liquid state or ina solid state. In one embodiment, the electrolyte has a viscosity of 10to 8,000 cps (centipoises), preferably 100-8,000 cps, more preferably1,000-8,000 cps.

The electrolyte may also comprise an additive such as an ionic liquid, alithium salt, or a pyridine derivative (such as 4-tert-butyl-pyridine).In one embodiment, the electrolyte comprises 0.01-1.0 M lithiumperchlorate (LiClO₄), preferably 0.05-0.5 M LiClO₄, more preferablyabout 0.1 M LiClO₄; 0.001-0.1 M lithium iodide (LiI), 0.005-0.05 M LiI,or about 0.01 M Li; and 0.1-100 mM iodine (I₂), preferably 0.5-10 mM I₂,or about 1 mM I₂, all in an acetonitrile (AN) solvent.

In one embodiment, the electrolyte may comprise a conductive polymer,such as polypyrrole, polyaniline, polythiophene,poly(3,4-ethylenedioxy-thiophene), poly(3-alkylthiophenes),polyacetylene, polyphenylene vinylene, and/or polyphenylene sulfide. Inanother embodiment, the electrolyte comprises or is made of acrosslinked polymer. A crosslinked polymer as used herein may refer to atype of polymers that are formed upon curing monomer resins (i.e.constituent units of a polymer) having a functionality of more than two(i.e. having more than two reactive sites) to form a three-dimensionalpolymer network structure that cannot be reprocessed into a differentshape upon heating without chemical degradation. According to thisembodiment, exemplary crosslinked polymers may include, but are notlimited to epoxy (e.g. an imidazole-curable epoxy with animidazole-based curing accelerator and a metal salt, or an amine-curableepoxy), polyester, vinylester, polyimide, polyamide-imide, polyurethane,polyphenol, poly(bis-maleimide), crosslinked rubbers, crosslinkedpolyvinyl alcohol, crosslinked polyethylene (e.g. crosslinkedhydrogels), nylon, polyhexahydrotriazine, polyisocyanurate,polyglycolide, polylactic acid, polycaprolactone, polyhydroxyalkanoate,polyhydroxybutyrate, polyethylene adipate, polybutylene succinate,poly(3-hydroxybutyrate-co-3-hydroxyvalerate), polyethyleneterephthalate, polybutylene terephthalate, polytrimethyleneterephthalate, and polyethylene naphthalate. In one embodiment, the term“crosslinked polymer” may also refer to a thermoplastic polymer whereinpolymers chains are physically bonded together (e.g. entanglement)without any covalent bonding therebetween. In a preferred embodiment,the crosslinked polymer has an average molecular weight betweencrosslinks in the range of 200-10,000 g/mol, preferably 2,000-5,000g/mol, more preferably 3,000-5,000 g/mol, even more preferably4,000-5,000 g/mol. The “average molecular weight between crosslinks”refers to an average molecular weight of a section of a polymer chainthat lies between two crosslinking points. In another preferredembodiment, a glass transition temperature of the crosslinked polymer isin the range of −50 to 100° C., preferably −50 to 50° C., morepreferably −50 to 0° C. In some preferred embodiments, the crosslinkedpolymer has a branch structure which makes it difficult to crystallize;the crosslinked polymer has a great interfacial adhesion with thesemiconductor layer and is capable of infiltrate pore structures in thesemiconductor layer; the crosslinked polymer has a great bondingstrength between the anode and the cathode; the crosslinked polymer iscapable of dissociating metal salts and transferring ions therethrough.An electrolyte solution having a dielectric constant within a range of1-200, preferably 1-100, more preferably 1-50 may be absorbed within thecrosslinked polymer.

In one embodiment, the method may further comprise a step of adding abarrier layer. The barrier layer may be added either before or after thedye, and may comprise compounds such as UV stabilizers, UV absorbingagents, and/or antioxidants. The barrier layer may improve theefficiency of the solar cell by converting UV irradiation into visiblelight, or may absorb UV irradiation to minimize dye degradation. In arelated embodiment, one or more UV filters may be placed against theoutside of the flexible DSSC, adjacent to the photoanode, the counterelectrode, or both.

In one embodiment, the sandwiching an electrolyte between the flexiblePt counter electrode and the flexible photoanode involves a step ofcoupling or connecting the counter electrode and the photoanode with anadhesive agent. The adhesive agent may be a thermoset polymer (e.g. UVor heat-curable epoxies, UV or heat-curable acrylates, orcyanoacrylates). Preferably, the adhesive may be a cyanoacrylate such asmethyl 2-cyanoacrylate (MCA), ethyl 2-cyanoacrylate (ECA, also known asKRAZY GLUE or SUPER GLUE), n-butyl cyanoacrylate (n-BCA), octylcyanoacrylate, or 2-octyl cyanoacrylate. Accordingly, a layer of theadhesive agent is placed between the anode and the cathode and pressure,heat, and/or UV irradiation is applied thereto. In the embodimentwherein the electrolyte is in a liquid state, one or more fine holes (orcavities) may be formed somewhere on the adhesive agent so that theelectrolyte may be injected into the gap between the photoanode andcounter electrode through said holes (or cavities). Further, an externalpart of said holes (or cavities) may then be sealed with an adhesiveagent. Alternatively, one or more holes may be made through thephotoanode or the counter electrode for injecting an electrolyte within,and the hole may have a diameter of 0.5-2 mm, or about 1 mm. In theembodiment wherein the electrolyte is in the solid state, theelectrolyte may be coated on the dye and metal oxide coated layer of thephotoanode. The photoanode may be coated with the electrolyte using aroll knife coater, a Gravure coater, a die coater, a reverse coater, orsome other application technique previously described. Then, theelectrolyte may optionally be dried or cured, and finally, the cathodeis disposed on the electrolyte via, for example, roll lamination. Inanother embodiment, the electrolyte may be first coated on the cathode,optionally dried or cured, and finally laminated on the anode. In oneembodiment, the electrolyte infiltrates to the dye and metal oxidecoated layer by applying heat. In another embodiment, rather than usingan adhesive or thermosetting compound, the electrolyte may be sealedbetween the photoanode and the counter electrode with a gasket or a ringof film. The edges of the two flexible substrates may entirely alignwith each other, or may have one or more edges that are overhanging ornot aligned. In one embodiment, the one or more edges of the flexiblesubstrates may overhang or not align in order to provide a structuremore suitable for handling, or to provide an area for connecting anelectrical conduit. An electrical conduit may be attached to the counterelectrode and photoanode by a clip or by soldering or using a conductiveadhesive.

By sandwiching the electrolyte, the electrolyte forms a first electricalcontact to the dye and the metal oxide particles of the photoanode, andthe electrolyte forms a second electrical contact to the platinumnanoparticles of the flexible Pt counter electrode. As defined here, twomaterials forming an electrical contact means that there exists a pathfor electrons to flow between the two materials, which path has anelectrical resistivity of at most 10⁻⁶ Ω·m, preferably at most 10⁻⁷ Ω·m,more preferably at most 10⁻⁸ Ω·m at a temperature of 20-25° C.

In one embodiment, the flexible DSSC has an open circuit voltage(V_(OC)) of 0.4-1.0 V, preferably 0.5-0.9 V, more preferably 0.6-0.9 V,even more preferably 0.68-0.75 V under solar irradiation or solarsimulator irradiation (AM1.5 conditions), and at front and/or rearillumination. The V_(OC) may be thought of as the voltage at a currentdensity of 0 mA/cm² in an I-V (current-voltage) curve, such as that inFIGS. 7A-7C. In one embodiment, V_(OC) from front illumination may beslightly larger than the V_(OC) from rear illumination. A ratio of thefront illumination V_(OC) to the rear illumination V_(OC) may range from1:1-1.10:1, preferably 1.01:1-1.07:1, or about 1.03.1:1.

A short circuit density may be thought of as the current density at 0 Vin an I-V curve, again shown in FIGS. 7A-7C. In one embodiment, theflexible DSSC has a short circuit current density (J_(SC)) under frontillumination of 5.0-17.0 mA/cm², preferably 6.0-12.0 mA/cm², morepreferably 7.0-9.0 mA/cm², or about 8.6 mA/cm², or about 8.7 mA/cm². Inone embodiment, the flexible DSSC has a short circuit current density(J_(SC)) under rear illumination of 5.0-10.0 mA/cm², preferably 6.0-9.0mA/cm², more preferably 6.5-8.0 mA/cm², or about 7.1 mA/cm², or about7.2 mA/cm². In one embodiment, a ratio of the rear illumination J_(SC)to the front illumination J_(SC) may be 0.50-1.00, preferably 0.60-0.90,more preferably 0.75-0.87, or about 0.82.

Another defining term in the overall behavior of a solar cell is thefill factor (FF). This factor is a measure of quality of a solar cell.This is the available power at the maximum power point divided by theopen circuit voltage (V_(OC)) and the short circuit current density(J_(SC)). In one embodiment, the flexible DSSC has a fill factor (FF) of45-90%, preferably 50-70%, more preferably 51-58%, or about 53% undersolar irradiation or solar simulator irradiation (AM1.5 conditions), andat front and/or rear illumination.

Illumination efficiency is the percentage fraction of incident radiationpower that is converted to electrical power by the solar cell. In oneembodiment, the flexible DSSC has a front illumination efficiency (η) of2.0-10.0%, preferably 2.5-7.5%, more preferably 3.0-5.0%, even morepreferably 3.1-3.5%, or about 3.2%, or about 3.3%, under solarirradiation or solar simulator irradiation (AM1.5 conditions).

In one embodiment, the flexible DSSC has a rear illumination efficiency(η) of 1.0-9.0%, preferably 1.5-6.0%, more preferably 2.0-4.0%, evenmore preferably 2.2-3.0%, or about 2.6% under solar irradiation or solarsimulator irradiation (AM1.5 conditions).

In one embodiment, the flexible DSSC has a percentage ratio (η_(R)) ofrear illumination efficiency to front illumination efficiency of 60-95%,preferably 65-85%, more preferably 70-82%, or about 80% under solarirradiation or solar simulator irradiation (AM1.5 conditions).

In one embodiment, where a solar simulator is used to produceirradiation, the radiation source may be a xenon arc lamp or a halogenlamp. In one embodiment, the radiation source may be fitted with adichroic reflector and/or an optical filter in order to better reproducesolar light illumination, such as illumination having an AM1.5Gspectrum, which may be known as the “global standard” spectrum.

The air mass coefficient (AM) defines the direct optical path lengththrough the Earth's atmosphere, expressed as a ratio relative to thepath length vertically upwards, i.e. at the zenith. The air masscoefficient can be used to help characterize the solar spectrum aftersolar radiation has traveled through the atmosphere. The air masscoefficient is commonly used to characterize the performance of solarcells under standardized conditions, and is often referred to using thesyntax “AM” followed by a number. The air mass number is dependent onthe Sun's elevation path through the sky and therefore varies with timeof day and with the passing seasons of the year, and with the latitudeof the observer. For characterizing terrestrial power-generating panels,the “AM1.5” standard is commonly used for illumination. “AM1.5”represents sunlight through a 1.5 atmosphere thickness, whichcorresponds to a solar zenith angle of 48.2°. The spectrum may besimilar to those defined by ASTM G-173 and IEC 60904 standards. In otherembodiments, the illumination may have an AM1.5D spectrum, which isknown as the “direct standard” spectrum. In alternative embodiments, adifferent illumination standard may be used, such as AM0, AM1, AM2, AM3,or AM38.

In one embodiment, the solar simulator output to the flexible DSSC tomeasure the above parameters (V_(OC), J_(SC), FF, illuminationefficiency, may be 90-110 mW/cm², preferably 95-105 mW/cm², morepreferably 98-102 mW/cm², or about 100 mW/cm².

Preferably, where the flexible DSSC is tested to measure the aboveparameters while under illumination, the temperature of the flexibleDSSC is kept from overheating, for instance, by a heat sink, a coolingblock, an air fan, or air conditioning. Preferably a temperature of theflexible DSSC is less than 50° C., preferably less than 45° C., morepreferably less than 40° C.

In one embodiment, the method may only consist of the steps necessaryfor making a flexible, Pt counter electrode. This procedure involves thesteps of depositing a solution comprising a platinum precursor and analcohol onto a conductive layer of a flexible substrate to produce acoated substrate and irradiating coated substrate with a UV lamp to formcrystalline platinum nanoparticles on the first substrate. The differentparameters and embodiments of these steps and materials have beendescribed previously.

The examples below are intended to further illustrate protocols forpreparing and characterizing the flexible bifacial dye-sensitized solarcell, and uses thereof, and are not intended to limit the scope of theclaims.

Example 1

Results

Platinum counter electrodes were photofabricated under differentfabrication parameters and characterized. Platinic acid precursor at aconcentration of 0.02 M in ethylene glycol (EG) was spin coated at 2000rpm for 30 s at a ramp rate of 500 rpm. 20 μL of the EG solution ofH₂PtCl₆ were deposited on an area of 0.25 cm² before spinning. Threesuccessive spin coating cycles were conducted to ensure proper adhesionof the Pt precursor solution on the FTO substrates at the chosen spincoating speed. After the spin coating cycles had been completed, SCOTCHtape used for the exposure of the coated area was removed beforetreatment with UV irradiation at ambient room conditions. On completionof the UV irradiation, the Pt precursor was reduced to Pt on the FTOsubstrate (FIG. 1A). The conversion was confirmed by differentcharacterizations reported in this work. To get proper insight into theUV conversion process and attaining advantageous parameters for thephotofabrication process, effect of irradiation time was studied. Theloading amount of the Pt precursor was also investigated by threedifferent spin coating cycles. Finally, we examined the versatility ofthe photofabrication techniques by using solvent other than EG (in thiscase ethanol), a drop coating method, and a flexible substrate(PET-ITO).

Effect of Irradiation Time

To study the effects of irradiation time, samples were treated withdifferent UV irradiation times of 2 h, 1 h, and 30 min. The preparedsamples with different irradiation times were characterized and used asCEs in the fabrication of DSSCs. FIG. 1B shows the transmittance spectraof the three samples and that of bare FTO glass. The visible lighttransmittance of the three samples are higher than that of bare FTOglass. For an understanding of the recorded enhancement of transmittancein the three samples, we examined the effect of UV irradiation on bareFTO glass. FIG. 1C shows the transmittance spectra of a pre-cleaned FTOglass that was treated with UV irradiation for different time intervals.The transmittance spectra were measured successively at 30 min, 1 h, and2 h UV irradiation times. We noticed dependence of transmittance spectraon irradiation time. With this observation, the effect of UV irradiationon the resistivity the FTO sample was investigated by measuring theresistance between two pre-marked points at 15 min, 30 min, 1 h, and 2 hUV irradiation time. The resistance of the UV treated FTO was found todecrease as the irradiation time increases, as shown in FIG. 1D.Meanwhile, the transmittance spectra of FIG. 1A suggest that the UVinteraction with the Pt precursor on the FTO glass and subsequentformation of Pt metal equally played part in the enhancement of thetransparency of the Pt EG-FTO CEs within the visible light region. Thesample that underwent 1 h UV irradiation photofabrication showed thehighest transmittance across the visible light region of 400 nm to 720nm. The samples that were treated for 30 min and 2 h had almost the sametransmittance spectra across the same wavelengths.

The photoreduction of the H₂PtCl₆.6H₂O in ethylene glycol (EG) to Ptmetal for the photofabricated CEs was investigated by XPS. The XPSspectra of the three samples with different UV irradiation times werecompared to understand the effect of UV irradiation time on thephotoreduction process. FIG. 1E shows the XPS survey spectra of thethree different samples with 3 cycles of spin coating and UV irradiationof 30 min, 1 h, and 2 h. All three samples exhibit a platinum peak at Pt4f orbital. A small chlorine peak at Cl 2p is observed to reduce in sizeas UV irradiation time is increased. For instance, at 1 h UV irradiationtime, the Cl 2p peak is much smaller than at a UV irradiation time of 30min, and is absent at 2 h UV irradiation time. FIG. 1F compares theplatinum peaks of the three samples at the respective UV irradiationtimes. These peaks might be due to degradation of the platinum coatingon the FTO. The Pt peak at 30 min UV irradiation exhibits a bindingenergy of the Pt 4f_(7/2) at 72.91 eV, which is shifted away from thepeaks at 71.77 eV and 71.48 eV for the 1 h and 2 h UV irradiation times,respectively. The Pt 4f_(7/2) binding energies of the of the 1 h and 2 hUV irradiation times are closest to the atomic platinum binding energyof 71.2 eV. The Pt peak at 2 h UV irradiation has a lower intensity peakthan those of 30 min and 1 h irradiation. Hence, the correct UVirradiation time for the photofabrication process is important.

The SEM images shown in FIGS. 2A, 2B, and 2C for the threephotofabricated Pt CEs with different UV irradiation times of 2 h, 1 h,and 30 min, respectively, indicate that Pt nanoparticles are welldispersed on the FTOs with no agglomeration visible on the morphology ofthe photofabricated Pt CEs.

The electrochemical characterization of the samples was carried out tostudy the catalytic activity of the samples in triiodide/iodideelectrolyte. FIG. 2D shows cyclic voltammograms (CV) of the samples. Thebare FTO sample shows no catalytic activity as no reduction or oxidationis present in the CV scan of the sample. UV irradiated Samples with 1 hand 2 h UV irradiation show both reduction and oxidation peaks that arealigned throughout the CV scan while the sample with 30 min UVirradiation exhibits a slight shift in the reduction and oxidationpeaks. CV measurement is particularly useful in understanding theregeneration of dye molecules from the triiodide/iodide electrolyteafter the photoreduction of the dye molecule in generation of electroninto the TiO₂ photoanode material, as redox equilibrium is desired forthe continuous functioning of the solar cells. The redox reaction at theelectrolyte/photofabricated Pt CE interface is as given in equation (1):I₃ ⁻+2e ⁻→3I⁻3I⁻→I₃ ⁻+2e ⁻  (1)The Nyquist impedance plot shown in FIG. 2E illustrates the chargetransfer mechanism between the electrolyte and photofabricated Pt CEs insymmetric dummy cells. The fitting of the Nyquist plots is carried outwithin the NOVA 2.1 software. The equivalent circuit used in fitting theNyquist plot is as shown in FIG. 2F. The series resistance R_(S), chargetransfer resistance R_(CT), constant phase element (CPE) and theexchange current density (J₀) of the dummy cells are summarized inTable 1. The sample with 1 h UV irradiation exhibits the least seriesresistance and charge transfer resistance of 17.641Ω and 10.639Ω,respectively. The 2 h UV irradiated sample also performed better thanthe 30 min UV irradiated sample. The surface area of the photofabricatedPt CEs as given by the CPE shows dependency with UV irradiation times(Table 1). The values of J₀ are obtained from equation (2):J ₀=(R·T)/(n·F·R _(CT))  (2)where R represents the molar gas constant, T (=298 K) is the absolutetemperature, n represents the number of electrons involved in thetriiodide reduction at the electrode/electrolyte interface having avalue of 2, and F is the Faraday constant. See Lan, Z., Wu et al.,incorporated herein by reference in its entirety. The Tafel plots forthese Pt CEs are as shown in FIG. 2G. Pt-EG-FTO UV irradiated for 1 hexhibited the highest value for both the anodic and cathodic currentdensities.

From the transmittance, XPS, and electrochemical characterizationsresults discussed above, we concluded that 30 min irradiation was notsufficient to reduce the Pt precursor to Pt metal. On the other hand,excessive UV exposure appears to be detrimental to the photofabricationprocess as is the case for the 2 h UV irradiated sample. Hence, 1 hirradiation time was used for the UV photofabrication Pt CE technique.

TABLE 1 Nyquist impedance parameters of photofabricated Pt CEs CPE J₀Photofabricated Pt-CEs R_(S) (Ω) R_(CT) (Ω) (μF) (mA · cm⁻²) UV 2 h(Pt-EG-FTO) 18.21 22.697 11.91 0.57 UV 1 h (Pt-EG-FTO) 17.64 10.639 7.091.20 UV 30 min (Pt-EG-FTO) 18.17 40.788 5.69 0.32 UV 1 h (Pt-EtOH-FTO)30.36 24.098 10.38 0.53 UV 30 min (Pt-EtOH-FTO) 37.65 143.53 9.20 0.09UV 15 min (Pt-EtOH-FTO) 22.17 241.04 5.58 0.05 UV 1 h (Pt-EtOH-PET-ITO)366.10 1655.30 0.39 0.008Effect of Solvents

The versatility of our photofabrication technique with respect todifferent solvents is reported. Here, we chose ethanol as arepresentative solvent of other suitable solvents that are used in thesynthesis of Pt metal from H₂PtCl₆.6H₂O precursor. Ethanol, being anontoxic solvent, has an advantage of a low boiling point of 78° C.compared to ethylene glycol (EG), and it can therefore evaporate moreeasily. Owing to this advantage, a drop casting method was used fordepositing the platinic acid in ethanol precursor for thephotofabrication process. The drop casting method utilized much lowerplatinic acid solution, leading to minimal Pt loading as compared tospin coating process that results in wastage of material. For the dropcasting process, 30 μL of 0.02 M solution of H₂PtCl₆.6H₂O in ethanol wasdropped onto an FTO glass having an exposed area of 0.25 cm². Threesamples were prepared using this approach and then UV irradiated for 1h, 30 min, or 15 min. FIG. 3A shows the transmittance spectra of thephotofabricated Pt CEs from the ethanolic platinic acid solution. Acrossthe entire visible light wavelength region, each of the three sampleshas a greater transmittance the bare FTO.

FIG. 3B shows the XPS spectra of the photofabricated Pt CEs from ethanolbased platinic acid solution. All three samples at different UVirradiation times show platinum peaks that are more prominent than thoseof the samples photofabricated from EG platinic acid solution. Thisindicates that the drop coated precursor samples have a better and moreefficient platinum loading than the spin coated samples. The effect ofUV irradiation time can as well be seen from the XPS spectra. As UVirradiation time increases from 15 min to 1 h, the platinum peaks can beseen to increase with respect to the irradiation time (FIG. 3C).Meanwhile, the chlorine peaks decrease as the UV irradiation timeincreases (FIG. 3B), confirming the photoreduction of the H₂PtCl₆.6H₂Oin ethanol to Pt. Pt-EtOH-FTO with 1 h of UV irradiation exhibited Pt4f_(7/2) peak at a binding energy of 71.27 eV, while Pt-EtOH-FTO with 30min and 15 min UV irradiation had a shifted Pt 4f_(7/2) peaks at bindingenergies of 72.81 and 73.62 eV, respectively.

The SEM images of photofabricated Pt CEs at different irradiation timesare presented in FIGS. 4A-4C. FIGS. 4A, 4B, and 4C show samples exposedto 1 h, 30 min, and 15 min UV irradiation, respectively. FIG. 4A showssimilarly well dispersed Pt particles on the FTO. However, FIG. 4C showsa different morphology of sheet and cloud-like structures indicatingthat the ethanolic platinic acid solution has only been partiallyphotoreduced, while FIG. 4B shows traces of the sheet and cloud-likestructures that are seen in FIG. 4C, further underscoring that 30 min UVirradiation was not sufficient for the photofabrication process.

The CV scan measurements of the photoreduced ethanolic platinic acidbased Pt CEs are presented in FIG. 4D. Consistent with the XPS spectraand SEM images, the 15 min UV irradiated sample showed poor catalyticactivity as it exhibits little reduction and oxidation peaks in the CVscan measurement. On the other hand, the 1 h UV irradiated sample showedmore prominent reduction and oxidation peaks, indicating very goodcatalytic activity. Pt CE photofabricated with 30 min UV irradiationequally manifested good catalytic activity.

The Nyquist plot parameters of all ethanolic based photofabricated PtCEs are presented in Table 1, and the Nyquist plots are shown in FIG.4E. Owing to the good Pt loading and high catalytic activity, the 1 h UVirradiated sample (Pt-EtOH-FTO) has a small series resistance and smallcharge transfer resistance. The Nyquist plots further confirmed theinsufficiency of 15 min irradiation time for the photofabricationprocess. The series and charge transfer resistances are to be higherthan all photofabricated Pt CEs. CPE follows similar dependency with UVirradiation time as that of Pt-EG-FTO CEs. The calculated J₀ values forthe Pt-EtOH-FTO are also listed in Table 1. Tafel plots in FIG. 4F showPt-EtOH-FTO with 1 h UV irradiation having the highest current densityvalues for the anodic and cathodic current densities as compared withthose of 30 and 15 min UV irradiation time.

Example 2

Photofabrication of Pt on PET-ITO

Flexible Pt CE on PET-ITO (Pt-EtOH-ITO-PET) was photofabricated as ademonstration of the versatility and potential area of application ofthe photofabrication technique. 5 μL ethanolic platinic acid precursorwas drop casted on an exposed area of 0.25 cm² of PET-ITO substrate andthen treated with 1 h UV irradiation. The ambient temperature did notexceed more than 40° C. during the UV irradiation, which makes itsuitable for use on flexible substrates. FIG. 5A is the transmittancespectra of the photofabricated Pt flexible CE. The obtained spectra showimprovement of the transparency of the photofabricated Pt flexible CE ascompared to bare PET-ITO substrate. This is consistent with thetransmittance results obtained for photofabricated Pt CEs on FTOsubstrates. FIG. 5B shows the SEM morphology image of the Pt flexible CEwith both well dispersed Pt and agglomerated sites. The catalyticactivities of photofabricated Pt flexible CE were investigated by CVscan, electrochemical impedance spectroscopy (EIS) Nyquist measurement,and Tafel plot. FIGS. 5C and 5D show the CV scan measurement and Nyquistplot, respectively, of the photofabricated Pt flexible CE. The Tafelplot is shown in FIG. 5E.

Example 3

Solar Cell Performance

The photofabricated Pt CEs were used for the fabrication of bifacialDSSCs (FIG. 1A), and their photovoltaic performances were measured underthe illumination of an AM1.5G solar simulator at 100 mW·cm⁻² lightintensity. FIG. 6A shows typical DSSCs fabricated using thephotofabricated Pt CEs while FIG. 6B shows the image of the fabricatedflexible DSSC (flex-DSSC) utilizing the photofabricated Pt flexible CE.The front and rear illuminated DSSCs I-V parameters are summarized inTable 2. FIG. 7A-7C illustrate the front illumination I-V curve (currentdensity vs. voltage) for the spin-coated (Pt-EG-FTO), ethanol based dropcoated (Pt-EtOH-FTO), and the flexible (Pt-EtOH-ITO-PET) CEs,respectively. FIGS. 8A-8C show the rear illumination I-V curves for theDSSCs fabricated with the respective Pt CEs. It is seen that the Pt CEswith 1 h UV irradiation time outperformed other Pt CEs with UVirradiation time other than 1 h for both EG and EtOH based fabricated PtCEs, respectively. The good catalytic activities of Pt CEs fabricatedwith 1 h UV irradiation time were further confirmed by electrochemicalCV, EIS, and Tafel characterizations. Solvent effect shows EtOH based PtCEs to be better and more efficient for DSSCs than EG based Pt CEs.Pt-EtOH-FTO CE with 1 h UV photoreduction exhibited the best performanceefficiency of 7.29% and the highest open circuit voltage (V_(OC)) of 810mV for front illumination. In a similar trend, DSSC utilizingPt-EtOH-FTO CE with 30 min UV irradiation performed better than allDSSCs fabricated with Pt-EG-FTO CEs with efficiency of 5.07% as comparedto the best EG based CEs of 5.01% (for UV 1 h (Pt-EG-FTO)). FlexibleDSSC based on UV 1 h (Pt-EtOH-ITO-PET) CE and employing ZnO as aphotoanode recorded a PCE of 3.26%.

All DSSCs employing photofabricated Pt CEs retained more than 77% oftheir front illumination efficiencies when illuminated from the rear.The percentage ratio of the rear illumination efficiency to the frontillumination efficiency (η_(R)) is given in Table 2. The η_(R) trend isconsistent with the reported transmittance spectra of thephotofabricated CEs. UV 2 h (Pt-EG-FTO) CE retained the highestpercentage conversion efficiency ratio at 85.92%, slightly above 85.42%of UV 1 h (Pt-EG-FTO) CE. Flexible DSSC recorded η_(R) of 79.75%. Hence,our photofabrication technique proved adequate for utilization inbifacial DSSCs. The difference in PCEs between front and rearillumination is observed to be largely due to the reduced photocurrentdensity of the rear illuminated DSSCs. This reduction in photocurrentdensity can be ascribed to electrolyte layer in the cell which behavedas a barrier between the incident light radiation and the dyesensitizer. The electrolyte is known to reflect incident light away,thereby reducing the amount of light available for the photoexcitationof the dye molecules.

FIG. 9A shows a picture of a typical transparent photofabricated Pt CE.FIGS. 9B and 9C both show a collection of four Pt CEs. The upper left,upper right, and bottom right samples of each figure are photofabricatedPt CEs, and the bottom left sample of each figure is the thermallyfabricated Pt CE. As a reference for comparison, thermally fabricated PtCE at 450° C. was used to fabricate DSSC. An efficiency of 7.54% wasrecorded slightly above the best photofabricated Pt CE DSSC. Thephotovoltaic parameters for this cell are listed in Table 2 for bothfront and rear illumination. FIG. 9D shows the front and rearillumination I-V curves for the DSSC fabricated with thermally preparedPt CE. The rear illumination photovoltaic performance significantlydeviates from the performance recorded for the front illumination. Thedeviation can be seen to result from the drastic drop in photocurrentdensity of the rear illuminated DSSC which consequently led tosignificant loss in fill factor. The high reflectance (lowtransmittance) of the thermally prepared Pt CE as shown in FIG. 9E isresponsible for the observed loss in photovoltaic parameters with asignificantly reduced PCE of 2.71%. A η_(R) of 35.94% was recorded forthis cell, which is 41.97% less than the photofabricated CE having aη_(R) of 77.91%.

As shown above, a novel photofabrication technique was developed for thefabrication of highly transparent Pt CEs with the aid of UV irradiationon rigid FTO glass and flexible PET-ITO substrates. The facile andversatile photofabrication technique was used to fabricate Pt CEs thatshow better transmittance across the visible light spectrum of 400 to700 nm wavelength than bare FTO glass and bare PET-ITO substrates. UVirradiation was found to improve both the transmittance and resistivityof bare FTO glass. Improved transmittance of photofabricated Pt CEs wasfound to be a function of increased UV irradiation time. XPS spectraconfirmed the photoreduction of H₂PtCl₆.6H₂O to Pt metal CEs. XPSresults established 1 h UV irradiation as an advantageousphotofabrication time. Catalytic activities of the photofabricated PtCEs studied by CV scan measurement, EIS, and Tafel plot are found todepend on UV irradiation time as complete photoreduction is necessaryfor a better catalytic performance. SEM images revealed well-dispersedPt nanoparticles on both the FTO and PET-ITO substrates. Investigationof solvent effects showed that ethanol as a volatile liquid is moresuitable for the photofabrication technique as minimal Pt precursor isused. Thus, ethanol saves material costs compared to EG based platinicacid precursors, which must be spin coated instead of drop casted. Spincoating deposition technique make use of more precursor material that islargely wasted making it cost ineffective. The photofabricated Pt CEswere used to fabricate bifacial DSSCs with PCEs attaining 7.29% forfront illumination and 5.85% for rear illumination as compare with DSSCutilizing thermally fabricated Pt CE having PCE of 7.54% and 2.71% forfront and rear illumination respectively. The highest percentage ratioof the rear illumination efficiency to the front illumination efficiency(η_(R)) was recorded as 85.92% while the lowest η_(R) is 77.91%.

TABLE 2 I-V characteristics parameters of photofabricated Pt CEs DSSCsV_(OC) FF η η_(R) Photofabricated Pt-CEs Illumination J_(SC) (mA · cm⁻²)(V) (%) (%) (%) UV 2 h (Pt—EG—FTO) Front 9.68 0.74 66.47 4.76 85.92 Rear8.32 0.74 66.43 4.09 UV 1 h (Pt—EG—FTO) Front 10.31 0.75 64.90 5.0185.42 Rear 8.82 0.75 65.15 4.28 UV 30 min (Pt—EG—FTO) Front 10.59 0.6938.49 2.81 79.36 Rear 8.57 0.67 38.88 2.23 UV 1 h (Pt—EtOH—FTO) Front13.53 0.81 66.56 7.29 80.25 Rear 10.89 0.802 67.03 5.85 UV 30 min(Pt—EtOH—FTO) Front 10.40 0.74 66.77 5.07 77.91 Rear 8.16 0.72 67.183.95 UV 15 min (Pt—EtOH—FTO) Front 9.79 0.75 53.10 3.92 78.57 Rear 7.930.74 52.62 3.08 UV 1 h (Pt—EtOH—PET— Front 8.67 0.71 53.35 3.26 79.75ITO) Rear 7.14 0.687 53.02 2.60 Pt—EtOH—FTO @ 450° C. Front 15.49 0.8457.73 7.54 35.94 Rear 7.225 0.759 49.51 2.71 *η_(R) is the percentageratio of the rear illumination to the front illumination.

Example 4

Materials and Method

Materials

Platinic (H₂PtCl₆) acid, ethylene glycol (EG) purriss grade, FTO glass(7 Ω·sq⁻¹) and ITO-PET (14·Ω·sq⁻¹). Lithium perchlorate (LiClO₄),iodine, and lithium iodide (LiI) were purchased from SIGMA ALDRICH.Acetone, 2-propanol, and methanol were purchased to from FISHERSCIENTIFIC. Ti-Nanoxide T/SP paste, and N719 sensitizer were allproducts of SOLARONIX, Switzerland. Iodide/triodide in acetylnitrile(AN) electrolyte was purchased from CHEMSOLARISM.

Platinum CE Photofabrication Process

FTO and ITO-PET substrates were cleaned successively using detergent,deionized (DI) water, acetone, and 2-propanol for 1 hour byultrasonication using BRANSON 3510. The substrates were heated at 70° C.for 20 min to completely remove the organic cleaning agents. 0.02 MH₂PtCl₆ solution in EG was prepared. The H₂PtCl₆ readily dissolved inthe EG solvent at room temperature. 20 μL of the platinic acid solutionwas then spin coated on the pre-cleaned FTO substrate with an exposedarea of 0.25 cm² at 2000 rpm for 45 s using SPECIALTY COATING SYSTEM(SCS) 6800 SPIN COATER SERIES. The H₂PtCl₆ solution coated FTO substratewas then exposed to UV irradiation using LUMEN DYNAMICS OMNICURE SERIES2000 at 2 W·cm⁻² at a distance of 5 cm from the platinic acid coated FTOsubstrate in an ambient environment for a specific duration of time. TheUV light intensity and temperature was measured by a homemadeARDUINO-based temperature sensor. The maximum temperature recorded at UVlight intensity of 2 W·cm⁻² was 40° C.

TiO₂ Photoanodes Fabrication

TiO₂ photoanodes were prepared by blade coating Ti-Nanoxide T/SP pasteon a 0.25 cm² exposed area of a pre-cleaned FTO glasses. The 0.25 cm²exposed area was achieved by covering the FTO glass with SCOTCH tapeleaving only an area of 0.25 cm² for coating of TiO₂ photoanode. TheTiO₂ coated FTO was heated at 450° C. for 30 min on a hot plate in openair. Prior to the heating, the masking SCOTCH tapes were removed. Afterthe TiO₂ paste had been baked for 30 min, the TiO₂ photoanodes wereallowed to cool down gradually to room temperature. The samples werethen soaked in N719 dye solution for 24 h.

ZnO Flexible Photoanode Fabrication

For the flexible DSSC, ZnO semiconducting photoanode was employed due toits low temperature processing potential. ZnO dispersion in butanol wasutilized for coating on ITO-PET. Prior to coating, the ZnO dispersionwas stirred for 2 h at 60° C. to achieve needed viscosity for apaste-like ZnO, which was then blade coated on ITO-PET and sintered at120° C. The sample was allowed to cool to room temperature before beingimmersed in N719 dye solution for 24 h.

N719 Dye Solution Preparation

N719 dye sensitizer was dissolved in methanol. The solution wassonicated for 30 min to dissolve the N719 dye. The prepared dye was usedto sensitize all TiO₂ photoanodes and ZnO flexible photoanode.

Dye-Sensitized Solar Cells Coupling

The DSSCs fabrication was completed by coupling the photoanodes and thephotofabricated Pt CEs using acrylic super glue gel. Triiodide/iodideelectrolyte was introduced into the cells before being hand-pressed toseal the electrolyte in between the electrodes and completing the cellsfabrication. The cells were left for some minutes prior to measuring theI-V characteristics.

Example 5

Characterization

XPS spectra of photofabricated CEs were carried out usingTHERMO-SCIENTIFIC ESCALAB-250Xi System equipped with monochromatic Al Kαradiation (hv=1486.6 eV). Spectra acquisition was done using a constantenergy mode with a pass energy of 100 eV and 30 eV for the survey andthe narrow scans, respectively. The analysis chamber base pressure was4×10⁻¹⁰ mbar. The photofabricated CE samples were mounted onto thesample holders with the aid of double-sided conductive adhesive tapesand outgassed in the sample loading chamber for 5 h at 2×10⁻⁷ mbar. TheXPS data acquisition was carried out using THERMO-SCIENTIFIC AVANTAGEsoftware.

The morphology of the fabricated samples was studied using a LYRA TESCANfield emission scanning electron microscopy (FESEM) equipped with anaccelerating voltage of 5 kV.

Transmittance spectra of the photofabricated samples were recorded usinga JASCO 670 double beam spectrophotometer at wavelength range of 400 nmand 900 nm.

To study the catalytic activity of photofabricated Pt CE samples, threeelectrode cyclic voltammetry (CV) measurements were conducted. Asaturated calomel electrode (SCE) served as the reference electrode, aplatinum plate sheet electrode was used as the counter electrode, andthe photofabricated Pt electrodes were placed as the working electrodesin the setup. The electrolyte contained 0.1 M lithium perchlorate(LiClO₄), 0.01 M lithium iodide (LiI) and 0.001 M iodine (I₂), all inacetonitrile (AN) solvent. The operating potential for the CVmeasurement ranged between −0.5 V and 1 V vs SCE. The CV measurement wascarried out on an AUTOLAB PG302N equipped with NOVA 2.1 software.

The electrochemical impedance spectroscopy (EIS) measurement was carriedout using AUTOLAB PG302N potentiostat equipped with NOVA 2.1 software.The Nyquist plot of the impedance parameters and Tafel plots werecarried out on the system. The operating frequency ranged from 0.1 Hz to100 kHz at a voltage scan rate of 10 mV/s.

The I-V characteristics of the photovoltaic performance of thefabricated DSSCs utilizing photofabricated Pt CEs were measured usingthe AUTOLAB PG302N potentiostat equipped with NOVA 1.11 software. AnORIEL lamp solar simulator calibrated to 100 mW·cm⁻² was used as thelight illumination source for the I-V characteristic measurement. Anarea of 0.25 cm² was exposed for the measurement.

As demonstrated above, Pt CEs were photofabricated with differentsolvents. The photofabrication process utilized UV irradiation ofdeposited solutions of H₂PtCl₆ to achieve Pt CEs. This novel method offabricating Pt CEs requires no pre/post-thermal treatment and wascarried out under ambient conditions. This method utilized minimal Ptloading and required no surfactant addition, which is usually requiredto be removed either by heating or other methods. Additionally, it issuitable for plastic substrates as no acidic treatment is performed inthe fabrication process.

The invention claimed is:
 1. A method of fabricating a flexible bifacialdye-sensitized solar cell, the method comprising: depositing a solutioncomprising a platinum precursor and an alcohol onto a first conductivelayer of a first flexible substrate to produce a coated substrate;irradiating the coated substrate with a UV lamp to form crystallineplatinum nanoparticles on the first substrate, thereby forming aflexible Pt counter electrode; separately applying a metal oxide on asecond conductive layer of a second flexible substrate to form a metaloxide coated substrate, which comprises metal oxide particles on thesecond conductive layer of the second substrate; depositing a dye on themetal oxide particles, thereby forming a flexible photoanode; andsandwiching an electrolyte between the flexible Pt counter electrode andthe flexible photoanode, wherein the electrolyte forms a firstelectrical contact to the dye and the metal oxide particles of thephotoanode, wherein the electrolyte forms a second electrical contact tothe platinum nanoparticles of the flexible Pt counter electrode, therebyfabricating the flexible bifacial dye-sensitized solar cell, and whereinthe Pt nanoparticles have an average diameter of 100-450 nm.
 2. Themethod of claim 1, wherein the Pt nanoparticles have a granular shape.3. The method of claim 1, wherein the platinum precursor is H₂PtCl₆,H₂PtCl₄, (NH₄)₂PtCl₆, or K₂PtCl₆.
 4. The method of claim 1, wherein thealcohol has a boiling point of 83° C. or lower at atmospheric pressure.5. The method of claim 4, wherein the alcohol is methanol, ethanol, orisopropanol.
 6. The method of claim 1, wherein the solution is depositedby dropping the solution onto the first conductive layer.
 7. The methodof claim 1, wherein the first conductive layer and the second conductivelayer are transparent and are each independently selected from the groupconsisting of ITO, FTO, AZO, GZO, IZO, IZTO, IAZO, IGZO, IGTO, and ATO.8. The method of claim 1, wherein the irradiating is done for a periodof 45-75 minutes and at an average intensity of 0.1-10 W·cm⁻².
 9. Themethod of claim 1, wherein the first flexible substrate and the secondflexible substrate each independently comprise a plastic selected fromthe group consisting of poly(ethylene terephthalate), poly(ethylenenaphthalate), polycarbonate, polypropylene, polyimide, triacetylcellulose, polyethersulfone, cyclo olefin copolymer, and polyarylite.10. The method of claim 1, wherein the irradiating does not raise asurface temperature of the first flexible substrate above 50° C.
 11. Themethod of claim 10, wherein the irradiating does not raise a surfacetemperature of the first flexible substrate above 40° C.
 12. The methodof claim 1, wherein the electrolyte comprises iodide.
 13. The method ofclaim 1, wherein the metal oxide is at least one selected from the groupconsisting of ZnO, TiO₂, SnO, Fe₂O₃, WO₃, CeO₂, BiVO₄, SrTiO₃, andBaTiO₃.
 14. The method of claim 13, wherein the metal oxide is ZnO. 15.The method of claim 1, wherein the flexible bifacial dye-sensitizedsolar cell has a percentage ratio of rear illumination efficiency tofront illumination efficiency of 45-95%.
 16. The method of claim 1,wherein the flexible bifacial dye-sensitized solar cell under AM1.5irradiation applied as front or rear illumination has an open circuitvoltage in a range of 0.4-1.0 V.
 17. The method of claim 1, wherein theflexible bifacial dye-sensitized solar cell under AM1.5 irradiationapplied as front illumination has a short circuit density of 5.0-17.0mA/cm².
 18. A method of fabricating a flexible bifacial dye-sensitizedsolar cell, the method comprising: depositing a solution comprising aplatinum precursor and an alcohol onto a first conductive layer of afirst flexible substrate to produce a coated substrate; irradiating thecoated substrate with a UV lamp to form crystalline platinumnanoparticles on the first substrate, thereby forming a flexible Ptcounter electrode; separately applying a metal oxide on a secondconductive layer of a second flexible substrate to form a metal oxidecoated substrate, which comprises metal oxide particles on the secondconductive layer of the second substrate; depositing a dye on the metaloxide particles, thereby forming a flexible photoanode; and sandwichingan electrolyte between the flexible Pt counter electrode and theflexible photoanode, wherein the electrolyte forms a first electricalcontact to the dye and the metal oxide particles of the photoanode,wherein the electrolyte forms a second electrical contact to theplatinum nanoparticles of the flexible Pt counter electrode, therebyfabricating the flexible bifacial dye-sensitized solar cell, and whereinthe metal oxide is applied in the form of a paste which is heated at atemperature of 100-150° C. to form the metal oxide coated substrate. 19.The method of claim 18, wherein the paste comprises butanol.